Declaring income earned in the UK

Hi, I am not sure if anyone will have an answer or some advice to give me. I am a French national who recently returned to France after living abroad for 46 years. I arrived in Nice on 27 August 2019. I have worked in several countries but never in France. Before coming back, I spent the last 12 years in London where I was successfully self-employed. There, I had an accountant to help me with my SA. My 2019 declaration d’impôts was easy enough since all I had to declare was my US pension (social security benefits) and income from a studio I am renting in France. 2020 will be more complicated as I still work part time with UK clients remotely. They continue to pay me in my UK bank account. Some people suggested that I register as an auto entrepreneur with URSSAF, others advised me against it. I wonder if I can declare my 2020 income as a lump sum - it’s not very much - and convert it from GBP to EUR using December 31st 2020 exchange rate. That is what I was told to do for my US pension for 2019. I would very much appreciate any insight you might have. Many thanks, Evelyne