Re: 25 billion fits easily into 500 billion
At which point we invoke the The Mundell-Fleming trilemma and the UK will be back begging the IMF for loans, as it did in the 1970s.
The mere prospect of the UK leaving the EU is already making funding the national debt more difficult and it is only going to get harder as the costs (lower tax receipts and more bureaucracy) rise. As a result there is going to be even less money available for investment than there already is (the UK compares poorly with other industrialised countries).
The current manipulation of the debt markets by the central banks disguises the fact that not all central banks have infinite balance sheets. As soon as it looks like any particular central bank is directly monetising government debt, then risk premiums will be applied. In the case of the UK, which is both able to issue debt in its own currency and force investors (insurance companies, pension funds and banks) to buy large amounts of debt, this may seem unlikely. But it's happened before and, as the recent failed auction hints at, it could easily happen again.
But I think that Corbyn and his Luddite Militants might actually be quite happy with rolling back technological change. Whereas I'd be content with just social media disappearing…