Sure that's possible. Use the Deutsche Bahn planner to check timetables. Do verify the UK timetables with the UK national planner, which will be more up to date. Do not use the rail planner app to plan: it doesn't get enough updates to be reliable. The fastest routes are usually via Brussels or Paris and the Eurostar to London (which requires a reservation of 30 EUR in 2nd class). An alternative is a ferry, for example Hoek van Holland - Harwich (with 30% Interrail discount). Hoek van Holland can be reached by metro from Rotterdam.

Visa requirements depend on nationality but EU citizens don't need one as far as I know. Do check with your foreign ministry to be sure. COVID requirements change regularly and with little notice; this page seems to have the current information for the UK.

Should have been much helpful if you had stated from where in DE and what nationality you have. EU-citizens now need a PASSpt (ID is not enough anymore) for normal tourist visits up to 1 or 3 monthes to UK. Also note-in case you come back, that there are very strict rules on what you can take with you from there-no dairy/meat/or in fact most food.

What you need for vaccin/tests etc depends again on when you go (I guess much later-so it will change a few times till then) and how old these are.

Check seat61. It is the site in /en/ that lists and describes all overland routeings from UK to anywhere in EUR-so read backward.

Thank you for your reply. I am not Eu citizen but i have a residence permit in Germany.

You mean from Germany to Brussels or Paris 

Brussels /Paris to London via Euro star. Is that correct? 

thank you