Working remotely in Germany

My UK employer will allow me work remotely 20 days a year abroad (I.e. with my company laptop logging into their secure network and company phone)

However, they have asked me to confirm that I have the right to work remotely in Germany.

Im finding it difficult to provide that confirmation in writing.

Can anyone point me in the right direction?

As a British citizen according to your profile (non-EU, since Brexit) you do NOT automatically have the right to work in Germany (whether remote or not does not matter from a legal standpoint). So you'd have to get a work visa first - and register as freelancer/self-employed, since you have no (German) employer, follow proper (German) accountancy rules, write bills for your services (incl. German VAT) and pay business and personal income taxes. This is not worth the effort for just 20 days a year. So better forget about the idea!

To add what bebbi has said, this will pretty much be the same answer for the rest of the EU as well.