Have you asked to convert your mortgage to buy to let mortgage or have you asked for “permission to let”?
Two very different things, the latter does not involve obtaining a new mortgage.
Can you…. go in and say “you are stuggling with the mortgage and want to move to a smaller place but cant sell. You are worried you are going to default on the mortgage and have considered handing them back the keys. If you had permisson to let that may solve things.”
They are obliged to help you and would always rather give permission than take a default. You would have to be able to back up your side with bank statements, accounts etc…
if you’re not in a tight financial spot then maybe they say yes, maybe they say no… but worth asking the question all the same.