I think plenty of lenders will do this sort of thing TBH.
The mortgage is secured against the property at the end of the day, so the lenders money is protected by that if you default. As long as the LTV gives the lender a good safety margin I believe plenty will do it.
I myself have an interest only mortgage at 40% LTV, and I used my investment assets to show that I had the funds to pay off the capital at the end of the term. I went straight to a high street lender, no middle man required. I think they had a rule about only recognising 80% of the sum invested - but as I had 3 or 4 times the loan it was no problem. I had to provide printed copies of ISA & SIPP statements.
I think the only issue may be demonstrating sufficient income to pay the interest over & above living costs. I am not quite sure how they treat pension income & income from investments - I did hear of a friend who had problems but was older.
Some silly policy rules seem to get applied in an attempt to protect the stupid and those that take on too much risk. But the bank have the property as security - in a less regulated world I don’t think they would bother with many other checks.
You need to plan for your own safety margins of course (interest rates could be triple in 5 years time) or you could end up like this if you over-use leverage...
https://www.cnbc.com/2021/09/08/...-and-heads-for-sale.html