The UK can only ask Germany to 'do something about it'. Courts apply to entities over which they have jurisdiction. UK isn't even in the EU anymore, they have no power over a German mail company.
The UK has no jurisdiction in Germany - they are separate sovereign states. It would be up to the UK to prevent them serving UK customers, because - as long as their own government dont interfere - they could just ignore any demands. This is just as true for the GDPR.
If you're operating in the UK, your operations there must, naturally, be subject to UK law.