Gregory's Server@JamesBaker @Edent And you don't, strictly, even need to do the declaration (although some people/organisations won't accept your name unless you do).
AFAIK the only true prohibition on simply using a name is if it's done with the intention to defraud.
@jack @JamesBaker @Edent Yep, the UK has no real concept of a ‘legal name’ in the same way as the US does. Names are just whatever you can convince people to call you.