Time was, in England, not that long ago, when you could use whatever name(s) you liked for whatever purpose(s) you liked, just by doing so. There was some sort of thing called a "deed poll" but that wasn't really necessary for anything much and didn't really add a lot.
And indeed I know a lady who changes her name every couple of years, simply by emailing round people and telling them her new name. However as she chooses increasingly unpronouncable names each time I've taken to calling her by the last name but three.
When we got married my wife changed one of her forenames (because her rather had spelt it wrong when registering the birth) simply by getting "formerly known as" written into the marriage register.
Times change, alas, and these days banks and airlines seem to think there's some sort of concept of someone's "real name", which is really a bit foreign to the English tradition. Ah well, at least we[#] got rid of the ID card bollocks, which would have set the concept of "real names" in stone, with your "real name" being dictated by a government computer, rather than by your choice of who you are.
[#] An achievement of Liberal Democrats in government.