I think you have to distinguish between UK residency and citizenship for an EU subject. With residence you are just declaring you are resident in a particular country and in most cases you then fall under their tax regime until you actually move elsewhere. Citizenship of course is renouncing your nationality and becoming British which is quite a step to take.

In Spain by being a resident I benefit from slightly different tax rates in some cases, am treated slightly differently should I sell my home here, and get discounts at some museums and stuff And its law that I have to register as a resident anyway. But in reality I dont really gain anything from it

But if you are a US national moving to the UK to work I'm guessing its going to be a whole different ball game as you will presumably be coming under the visa system