There are so many issues with defence spending that afflict not just the UK, but the west generally. But the business model is the biggest failure. It's "cost plus", meaning that it is in the interests of the manufacturers to make each item as expensive as possible. 30% profit on a million dollar missile that's hand-built is far better than 30% profit on a bunch of £50,000 ones.

They always want to sell "the latest greatest thing", knowing they can inflate the development costs hugely before delivering any actual systems. Then when they've built a few (always about 20% of the number originally quoted for, as penny-pinching ministers balk at the total cost and don't look at the amortised development). Once they've built what the politicians have funded, the production line is scrapped and the skilled workers have to go and find other jobs. Then the whole cycle starts again with the next "latest greatest" over-complex thing.

Compare that with Russia or China, where the private companies are in reality loosely disguised state enterprises. They'll pick a "good enough" system designed for a role, not by international committee. Then they'll set up a production line with a capacity of hundreds of units a month. It'll turn out maybe two or three a month for years, building up a cadre of trained workers. Then, when there is a need, they can spin it up to full production in a couple of months with the skilled workers training the new intake on weapons that are simple enough not to need hand-building. It's a totally different mindset.