Thanks for your question
Please advise where you are paid from (UK or Germany) and confirm this will continue.
I am assuming you are paid from Germany - in which case, as you will remit this income to the Uk and will actually now live in the UK, you will have a UK tax position that needs considering.
HMRC will therefore each year end need you to complete a self assessment tax return declaring the German income and any German tax suffered, and a furtehr charge on tax will only arise should our tax position be higher than one you have suffered. (as per double taxation treaty between Uk and Germany)
As far as your social security contributions are concerned, you will not have a UK liability (National Insurance) as the income is not UK arising but as some our our National Insurance does contribute towards NHS medical care - you would need to look furtehr into whether your German contributions cover you for UK care (we are tax and National Insurance experts but not in the NHS side of matters)
But I found this link here which may be of use https://www.citizensadvice.org.uk/healthcare/help-with-health-costs/nhs-charges-for-people-from-abroad/
But whether you pay UK national Insurance and then provide a certificate to you German employer detailing the fact you pay UK charges, might be an option to consider.
It may be that your German charges suffice but I would encourage you to take further steps to establish your position