Re: Who's opinion would you trust most out of this lot?
Brexit is only priced in to a certain extent. I think the current prices reflect an assumption that Britain will leave with some sort of deal with Europe, limiting the potential financial damage. Given that European politics often ends up with deals being done at the last minute we won't see the pricing in of a hard brexit until much closer to the trigger point, so a sudden drop causing a run on the pound isn't unthinkable.
House prices could also fall if there was a sudden net migration of people from the UK, lowering demand for housing. A hard brexit which removed rights to remain from EU citizens could provde that scenario - sure, some EU-based brits would come back, but their numbers would be fewer than those leaving.
I wouldn't be too sure about BTL landlords putting a backstop on the effects of declining property prices. For a long time rents have been hovering at around the same amount as the finance costs of a 100% loan on the property. When you factor in regulatory overheads (gas safety checks, legionnaires assessments, EPCs), agency/management fees, and property maintainance costs it's not unusual for BTL to barely break even or actually make a loss, especially as it's no longer possible to reclaim the full costs of financing (mortgage interest).
The whole industry has been riding on the idea that property values are forever going upwards, and that eventually returns will be realised through the capital gain on the properties. If the market does go into reverse it'll at the very least disrupt these plans for BTL investors, but if the expectation is for a longer period of decline it could trigger a further rush to sell, to get out while there's still some gain left. This could actually make the problem worse.
Wealthy foreigners might see UK property being effectively cheaper after a sterling crash, but if the outlook for those properties is further price falls they're not going to buy.
The trouble with doom and gloom scenarios is that the housing market isn't very liquid - transaction costs are enormous (stamp duty when buying, capital gains tax if selling BTL), and the cost of housing is now so high that making high percentage losses on property could bankrupt a significant proportion of the population. So the choices may instead be:
a) Sell quick and make a loss, which bankrupts you due to the resulting negative equity
b) Stay put and make it the lender's problem - worst case they can evict you and you're no worse off than a), but in the meantime you get a few months free accomodation while the lender gets the legal paperwork done.
The result of a falling market generally is that a lot of people hang on to their property, sales go down, and the whole market stagnates waiting for better times. The property market in London already seems to have slowed significantly - pretty much since the brexit result - which suggests to me that people are already spooked by the prospect of a long-term flat or declining market and aren't willing to buy in or cash out until things are resolved either way. If we get a good brexit outcome we might see house prices recover a little as business goes back to usual, if we see a bad one I'd expect stagnation to become more entrenched rather than seeing any sudden crashes.