Hi
marcovanbaI do think it's likely that in this situation you are probably overthinking things somewhat - it is not nearly as uncommon as you might think for males to change their name - in certain cultures for example, the male partner in a marriage will often change his name. There are also many other completely valid reasons to change a name - adoption, being estranged from birth parents, or simply not liking your name even. There are no legal restrictions on changing your name.
In terms of what financial institutions might think, I wouldn't be worried about this. Financial institutions do not have the time to thoroughly investigate every single customer who changes their name, and even if they did, the financial institution would have no legal (DPA-compliant) way to find out about your spent conviction.
You don't have to give the bank a reason for changing your name. If it were me in this situation, I'd simply go ahead, get the deed poll, and take it directly into a bank branch and ask them to update your details. They
might ask why you're changing your name, but I'd expect this would probably only be out of courtesy on the part of the employee dealing with you, to make conversation, rather than that information being stored anywhere. But in any event, you don't
have to answer them - again, if it were me, I'd just give a very non-committal answer (say something like I work in a public-facing role and am concerned about my privacy).
Anecdotally, I've heard from a few civil servants/other Government agency employees who have changed their names after leaving their service, so as to "disassociate" themselves from their previous public-facing role. Again, changing a name is not a particularly uncommon event, even for people who don't have a conviction.