The government props up the market, because otherwise they'll get booted out by the millions who have staked their retirement on property, plus the taxpayer owns £ billions of equity via the help to buy scheme, so the govt takes a direct hit of prices go down.
Local homeowners prop it up by ensuring no new housing gets built near them.
On the monetary side, the bank of England props up the market (e.g. funding for lending scheme), because our economy couldn't handle a crash, our banks are loaded to the gills with mortgage debt, they don't have enough capital to withstand a crash.
I can only see three scenarios where prices fall materially:
a) big interest rate hike (I believe this caused the 89 crash)
b) massive increase in homebuilding (250k+ p/a, sustained)
c) mass emigration out of the UK.
I really doubt a) will happen, the BOE is loathe to raise rates as our household debt is 90% of GDP, twice what it was in 1990, our economy wouldn't handle it, our banks would be insolvent as they have <5% capital. Inflation would need to be double digits before the BOE's hand is forced.
b) also very unlikely. Homeowners make up the biggest voting block, a huge vested interest group. The green belt is a sacred cow. Every time the govt hints at liberalising the planning system, the mail+telegraph kick up a 'hands off our land' campaign.
c) maybe if brexit goes really badly, but we'd be in a true economic catastrophe if this happens. Probably wouldn't have a job with which to pay for cheaper housing.
I'd say it's inevitable, the only question is when.
With the current unprecedented low interest rate levels it is hard to imagine, but there was a time that mortgage rates were above 10%. And there is no fundamental reason why they couldn't go back to those levels one day.