At the risk of keeping parts of this debate unnecessarily open, I think there is one important concept that is being misunderstood or misconstrued within Monzo (albeit probably in good faith):
As has been said by others, in England and Wales (and a lot of countries with common law) there is no concept of a “legal name”. Or more accurately any name a person uses legitimately (i.e. not for fraudulent purposes) is “legal”. Or to be absolutely exact, a person’s name is a matter of fact and useage, not a matter of law or formal registration.
So when @RichardR says (as above) there are some rules or regulations that require Monzo to communicate using a “legal” name, then either that requirement is based on a bogus and incorrect concept that such a canonical legal name exists, or it has been misconstrued somewhere down the line (not by Richard, but by a regulator or processing body perhaps).
[Edit: insertion for clarity] To address this in the terms Monzo use in their document quoted at the top of this thread, it is this statement that I say is flawed:
“There are many circumstances in which someone would prefer to use a name other than their legal name, for example:…”
In fact there ought be no distinction. Any name used becomes, by sole dint of its use, that person’s “legal” name. A so-called “preferred” name is a perfect example of a “legal” name because it is commonly used and established.[/Edit]
For instance, the requirement might be to quote the “name” that was recorded in the official identity documents seen by the bank. That would be an unfortunate and unhelpful requirement based on a misunderstanding of what a identity document establishes, and to use that to refer to a person further down the line a further reinforcement of the misconception.
We need to know what organisation is imposing the requirement for a “legal” name to be used in order to unpick where the error is. In the meantime we might persuade Monzo to take a more sensible attitude to a potentially nonsensical requirement.
There is no escaping the fact that for some bodies it is easier to play their games and provide them with strictly unnecessary paperwork, than it is to argue the toss at every juncture, but it doesn’t make it right.
Incidentally, it might be helpful to think of this in terms of changed surnames rather than as changed forenames as it is a concept we are all more familiar with in practice. For instance, it would be unconscionable for a bank to continue using a person’s married name, if they had become divorced and disassociated themselves from their married name by stopping using it, whether or not their passport had been updated yet.