they would most definitely repossess all of the houses where the owner will not pay the mortgage. if no one then bought them again, it would definitely cause a crash on the money markets & ruin the economy.
i live in the USA & do not know what lending practices r like in the UK. but i do know that if the USA money markets collapsed (and UK is investments here r a part of our money market) that it would lead to a worldwide depression. that is because all of the money markets in the world r dependant on what happens with ours. we also hold money & assets of different governments (which is why we can impose monetary sanctions on nations such as north korea if they displease us), & we lend out a huge number of dollars to other coutries for development in one way or another.
the way (here in the USA) that lenders r able to give new mortgages is by packaging up the mortgages they already gave into a portfolio which is sold out to investors in the secondary morgage market (pronounced like, but not necessarily spelled this way) such as fanniemae, ginniemae, & freddiemac. the portfolios r sold at a small discount so that the investors funnel money into the lenders & the cycle continues.
if all mortgagors discontinued to pay their principal & interest payments (or even just interest), nobody would want to buy mortgage portfolios & therefore no money would go to the lenders to fund mortgages. foreclosed properties may even just become straight rentals held by lenders, if nobody was willing to buy them.
however, in the case u mention, the value of the real estate would have to go down & down & down so far that properties would be getting sold so cheaply--far less than what the mortgage balances were--as to cause an entire monetary collapse & depression, not recession. because, even when people r not buying & selling real estate but simply refinancing it, we end up living in a recession. that is how important real estate SALES are: they fuel almost all our economy.
i wrote & coded a webpage one time on the real estate industry in the USA, getting data from the last census. when u look at how a hole in the ground eventually becomes improved real estate, considering all elements, including all labor, that go into making the building(s), sales, & closings, u realize that about 28-30% of our gross domestic product (GDP) is related to real estate! gee, if it were to get hit as u mention, all dominos would fall down & i wonder how we would survive? personally i think it could become a complete collapse of society & war in the streets.