I am thinking of moving back to UK-To Braintree Es-I married an American-I am now 73yrs. old?

I would like to move into a retirement flat. I am now an American citizen. What would I have to do to move back after all the years I have been away. I have been back almost every year since I left when I was 19 yrs. old. I recieve a small pension from UK plus SS from US. Do I just move over there? No problem, or is there some proceedures I have to follow? What are all the legalities? Just any input would be appreciated.