No idea how it is in practice, sorry.
But in the thread
UK a virus variant area it was mentioned just a few days ago that the website of the Germany embassy in the UK was changed and now explicitly disallows transit by car,
https://uk.diplo.de/uk-de/coronaviru...431868-5-panel
perhaps ask the Bundespolizei via this form for clarification?
https://www.bundespolizei.de/Web/DE/...rage_node.html