This may be a UK thing.
My husband and I bought a house just over 20 years ago in his country, where I am also now a citizen (dual nationality - thanks Brexit).
The mortgage was in both our names (strange as he had been out of work for a while and the numbers were actually my earnings), BUT only my name was on the deeds and I was the only one to sign the purchase contract.
When he died (he committed suicide just over a year after we bought the house) nothing around the mortgage changed - except that there was no big payout due to suicide. However, his ex was on to the land registry pdq to see if their son was entitled to half "my" house. He wasn't as his father was not on the deeds.
So I am just asking, because I have never bought in the UK, why are the names on the mortgage more important than the names on the deeds?