Then it was "The Few" - today it should be a few more

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"You don't have to be a pilot to fly in the RAF," intones the serious, throaty voiceover in the latest TV recruitment ad for the UK's Royal Air Force. And it is correct: there are plenty of other jobs - caterers, air traffic controllers, medics, office staff, drivers, dog handlers, instructors, even sign painters and gardeners.

You name it, and there is a job for you. Just don't expect 'pilot' to be one of them. Because to become a RAF pilot - especially a fast jet pilot - requires there to be planes that you can, you know, fly. Which, these days, is becoming a challenge.

By the end of this decade, the RAF will have the smallest number of frontline combat aircraft since its inauguration in 1918. By 2020 it is expected to have just 127 fighter jets available, as its remaining Tornados - a plane conceived in the 1960s and now operationally in its 40s - are retired along with, incredibly, the first "tranche" of its Eurofighter Typhoon jets, which were only introduced in 2003.

The Ministry of Defence and the RAF do, of course, maintain that they will be ready and able to deal with any global threat with what they have left - presumably a collection of rubber bands from the supply cupboard, a few redundant pilots wielding pitchforks, and the insanely over-budget and, apparently, inadequate F-35 Lightning II.

Famously, the last time the UK had to defend its airspace in anger was in the summer of 1940, the Battle of Britain. Then, "the Few" - RAF pilots, which included British as well as Czech, Polish, French and Canadian personnel - bravely held off Adolf Hitler's attempt to gain air superiority as a precursor to invasion.

Exactly 75 years ago today, August 18, 1940, the RAF experienced what became known as "The Hardest Day". Wave after wave of Nazi Luftwaffe planes were sent to attack key south-east England airfields, like Kenley, North Weald, Hornchurch and Biggin Hill, hoping to catch as many RAF aircraft on the ground as possible.

Mistakenly, the Germans had estimated that the RAF only had 300 fighters at its disposal, when in fact the reality was more than twice than that. Still, the odds were stacked heavily against the defending forces.

According to an RAF infographic posted on Twitter, in July 1940 the Luftwaffe had 864 fighters and 1,137 bombers, whereas the RAF had just 656 fighters, which included the fast new Spitfire, the slower Hawker Hurricane (although it ended the war with the greater number of 'kills') and even the Gloster Gladiator biplane.

By its end, The Hardest Day had produced some of the heaviest losses of the Battle of Britain on either side - some 36 RAF fighter aircraft destroyed in the air, 29 on the ground, and 63 damaged either in the air or on the ground, with 29 casualties including 10 killed. The Luftwaffe suffered even more, by as much as a ratio of 2:1.

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Comparing August 18, 1940 with August 18, 2015, is, of course, like comparing a Roman chariot with a Ferrari. Hurricanes and Spitfires were worlds apart in terms of capability to today's Typhoon, not to mention what goes into building it. It took ten years, from manufacturing contract to delivery, to produce the first production version of the £120 million-apiece Typhoon.

The Spitfire, on the other hand, took just three years from prototype to operational service, with the largest Spitfire factory - Castle Bromwich - producing more than 300 a month at the peak of production. Each Spitfire cost around £12,000, or roughly £200,000 at modern prices. By the end of its service life, 20,000 had been built.

Of course, World War II required a different national economy. At the end of the conflict, the UK was in a perilous financial state, and food rationing continued well into the 1950s.

It would be naive to compare wartime defence spending to today, but look around the world - be it Putin's Russia or the chaos in Iraq, Syria and Libya - and you have to ask yourself: are we properly protected? Could the RAF hold off another invader on the same scale as it did in 1940?

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In total, the RAF has today just under 800 aircraft of different types, but of these, less than 200 or so - the Typhoons and Tornados - are designed for frontline combat. But at a time of increased sabre-rattling by Russia (including a significant spike in the number of Russian planes testing UK air defences each week). the shrinking Royal Air Force - the oldest independent air force in the world - is stretched like never before.

Pilots are in short supply, squadrons are relying on spare parts from other air forces. Planes scheduled for the scrapheap are being given stays of execution to fill operational gaps while others are being sold or mothballed long before it is necessary to keep costs down. The RAF is, according to Chief of the Defence Staff General Sir Nicholas Houghton, at the "very limits of its fast jet availability".

Don't get me wrong: this post is not a conservative endorsement of warfare, a hawkish ambition to see more bomber aircraft instead of more nurses or even some anachronistic and jingoistic desire to see the UK return to its former glories. But for an island nation that last came under the very real threat of invasion 75 years ago, it is troubling that Britain should be placing itself at such comparative risk.

As has been well documented, procurement and project management in the Ministry of Defence has been a shambles: within the next couple of years there will be two new aircraft carriers with no planes to fly from them; even today, RAF aircrew are embedded with foreign air forces because the UK doesn't have its own planes, in strategically critical areas like anti-submarine reconnaissance, in which they can train.

During Operation Ellamy, the 2011 air campaign over Libya, flying instructors were called off training squadrons to fly jets on operations over North Africa. Back in June this year, the UK's public spending watchdog, the National Audit Office, revealed that a controversial £3.2 billion, 25-year project to outsource the training of all British military pilots is nearly six years behind schedule. Delays have meant that it will not be operational until 2019, placing further strain on the throughput of pilots to the RAF, Royal Navy, Royal Marines and Army Air Corps. And by the time they get there, what will they have to fly?

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In its last budget review, the UK has, at least, agreed to keep defence spending at 2% of its GDP, though it would be flawed to use this as a basis of comparison with others. US defence spending is way out on top at 3.5% of national output - a staggering $581 billion per year. But based on GDP, the UK is just above Greece, which is absurd. In reality, the picture of actual defence spending is more acute: according to the International Institute for Strategic Studies, the top ten defence spenders are, in reverse order, South Korea, Germany, India, Japan, France, the UK, Russia, Saudi Arabia, China and the United States, the latter of which has more combat aircraft than it knows what to do with them.

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Pilots will tell you that the Typhoon - now the backbone of Britain's air defence - is a very capable plane. 80 or so currently in service might sound a lot, and modern air defence does not compare with that of the 1940s. 'Ack-ack' guns and barrage balloons have given way to supersonic missile technology; the Spitfire - still described today as "pure" flying - required pilots to engage their enemy like hand-to-hand fighting in the air, whereas the Typhoon can dispatch a target from distance at the touch of a button.

But what would happen if an aggressor decided to take on the RAF's current frontline combat jets in numbers and succeeded? How long would it take to replace the pilots and their wings? It certainly wouldn't be another month before 300 new planes would be flying.