Owning your own home

Bit of cognitive dissonance in your post. If housing is a bad investment (ie BTLs are no longer profitable with a mortgage), then why would you buy a house, it makes no difference whether it’s your own or someone else’s. Rent and divert the savings from mortgage principal and maintenance into another asset class.

That makes no sense.

You have to live somewhere.
You can either buy or rent and they cost about the same per month.
So you have to put, say, £1500 a month somewhere.
You can either put it into a mortgage where £150 goes to interest and £1350 is effectively saved.
Or you can put it to rent, where 0 is saved.

BTL assumes you already have a property and you've got extra money hanging around that you can choose to either invest in a 2nd property or another asset class.
Sure, you can do BTL and get 5% return on it, or choose shares and get 7.5% return. But that's on property 2 onward. The same argument doesn't apply to your first property.

(If you can convince me otherwise then I'm definitely all ears, since I'm completing on my house sale in 3 days and am about to rent for a short while.)

Rent is generally more than mortgage.
For instance, my last place was 950 mortgage but was rented for a bit at 1400 odd.
So options are...
Pay more to not own a place or pay less and own a place, regardless of prices going up or even down.
If you need a better example, get £1500 pounds in cash and give it to a random stranger in the street, take another £1500 pounds in cash and pop it in your back pocket.
Guess which is renting and which one is buying in that metaphor.
BTLs are only profitable if you have a large amount of cash as mortgage repayments are no longer taken into account on tax on earnings, so if you get £1500 rent but have a £1500 mortgage you’ll actually be down each month. I assume folks with two properties are higher earners meaning you’ll be in the higher tax bracket so down near £700 a month.
Unless I’ve missed something? I’m usually pretty bad at this kinda thing...