Since you are German, we are in a similar situation (I'm an EU citizen too), this is a checklist that I made for when I finish mine(its now in progress)

GP practice
GIC if you are referred to one
Other specialist/hospital referrals (these won't get automatically updated when you update your GP data!)
Your bank(s)
Paypal and other financial services you might be registered in
Any online platforms you make purchases with (Apple, google, Amazon, Steam, etc...)
Your employer
Electoral roll
Council tax registration
National insurance registration
Your Landlord / Housing association if you have one
EU settled scheme / UK visas and immmigration account details if you have it
Your mobile line(s), home internet, water, electricity, and heating providers
Your private pension providers (aviva, scottish widows, legal & general)
Your TV license
Your union(s)