Yes, your residence card (Aufenthaltstitel) allows you to travel within Germany, but does not allow you to cross the national border.
You are, however, required to show and/or hand out your passport and/or residence permit to an authorized official such as
police and customs officer
government employees, such as foreigners and registration officials
You need your passport https://www.gov.uk/uk-border-control/before-you-leave-for-the-uk As a US citizen, residence in another EU country doesn’t give you the right to enter the UK without your passport.