Advice - Employed in the UK and working in Germany

Such international taxation issued are regulated in bilateral tax treaties.
I don't know the UK-Germany-treaty, but you should check it!
In general, such treaties give the taxation right to the country where the work is done, not where the money is coming from or paid out. That means you'll have to pay German taxes, if you live in Germany (in both jobs you mentioned) - PLUS: you will possibly have to submit both German and UK tax declarations (a complication I would rather avoid).
The idea of saving on taxes by such simple means is naive and will not work!

...Do not omit the effect of Brexit on the UK company employing you who will also have an increased tax burden as a 3rd country, since UK declined free movement of people and services. In all likelihood the old corporate tax tricks wont work anymore:
There was the tax praxis of overinflating subsiduary costs (ie Berlin) and shifting profits to head-office / tax „haven“ London).

Seeing how slow things dawn on some people with sandwiches crossing the border, it could be that their management realize effects only near June, they cannot overstate costs anymore, where even 100% subsidiary margin is not sufficient to offset the fat head-office anymore. The usual next step (UK cultural approach) is to „cut the legs“ = consolidate subsidiaries. Swedish management style is to reduce management overhead in subs, btw.

I guess you will be an employee, and with the UK consolidation style - For you personally it might mean staff reduction in Berlin or transfer to the left over subsidiary in another city. I‘d be careful if the business were not setup after 31 Dec 2020, where they recalc’ed fundamental profit/loss.

From accounting perspective this is the effect we can see on existing UK subsidiaries:

https://home.kpmg/us/en/home/insights/2 … tives.html

If you are a highly paid expert that can find an alternative job in Berlin quickly, I would say enjoy the ride and shovel their cream until they wake up.