Good point about interest rates. The other possibility is if Government policy turns significant against buy to let - for instance if Labor were elected on their current policy of making landlords sell at a discount to tenants. The elimination of buy to let demand in the market could reduce prices. Admittedly quite unlikely because most MPs are landlords.
As a BTL Landlord, this is a scenario we are all aware could happen.
However the small BTL landlords moving out of the sector will not create one single extra house nationally.
It will be the same number of houses, it may just give first time buyers a greater selection of houses to buy.
If it does force down prices, any drop would only be local and temporary, as the bottom line is there are simply more people than properties.
However if Brexit causes the UK economy growth to be noticeably lower than our neighbours on the mainland, then there will be a slow move towards the exit.
Many who have been here for decades will leave, those who would have come here will go to the neighbours instead, and most importantly the UK youth currently in education will leave for proper well paying jobs in Germany and France and many will never return.
That will then cause a fall in house prices as the number of people vs the number of properties required will reach equilibrium, and then a surplus in certain areas of the UK as the youth leave.