The only way I can see house prices crashes is if there something that causes a huge oversupply of houses vs demand:
-a huge epidemic/plague that kills off so many people that there are just a lot less buyers (of course the whole country would go down the pan),
-martial law that states anyone not British must leave the country and the houses will be sold off for peanuts (yay - we can all live in Belgravia/Mayfair)
-stock market/bank/insurance/pension obliteration (not just a crash) - but then you might not have the cash to buy because it is in the bank that went bust!
-the north pole shifting suddenly to Leicester (I picked that randomly),
-war against UK with a 'Blitz',
-an active volcano appearing in the Thames
Since none of these are likely to happen - I can't see how prices can crash to affordable levels - it would take a 50% reduction for homes to be affordable. Even in 2008 they didn't drop so much - around 15-16%.
The only other option is for wages/salaries to triple/quadruple which is just as unlikely.