Hi SarahLKK,
As you are resident in the UK, but working for a German employer, you will be taxable on your employment income in the UK and not in Germany. You will need to request a certificate of residence here:
How to apply for a certificate of residence to claim tax relief abroad
Article 14 of the UK/ German tax treaty covers employment.
You will then have to send the certificate of residence to the German tax authorities, along with any form they would require completion, so that they can repay any overpaid tax and ensure not further tax is deducted from your employment income.
You will be required to declare the foreign employment income in a Self Assessment tax return every year.
If you want to pay class 1 national insurance on your employment income, you will need to contact our employers helpline here for advice:
Employers: general enquiries
Thank you