The problem is that prices won't crash unless buying demand AND rental demand are both low. The reason goes back to the free markets theory - if rents are rising because of lack of supply, there will come a point when the rental yields of the property approaches high enough that investors will start buying those properties and converting them into rentals.
So unless supply in the rental market eases up and rents start to fall as well, it will keep supporting property prices at some level. Rents in London have gone up about 10% over the past year or so, and judging by the lack of rentals on the market and increase in letting agents around, it seems like rents might keep increasing - thereby making property a more attractive proposition to BTL investors.
There's at least two problems with that argument:
1) There's a perfectly efficient market in property
2) The level of yield
I think there's a more efficient market in flats. They get bought and rented out for the income stream, sure.
But I sincerely doubt many 3-4-5 bed houses are being bought for the purpose of being rented out in SE21/4. The yield is laughably low, unless they're not factoring in upkeep and voids.