Can you tell me what you do for the UK company exactly and where you do it please?
There is no need to dislcose the income to the UK tax authorities. You are working on a self-employed basis from Germany and the income will be taxable there. Article 7 of the UK/Germany tax treaty here deals with business profits. You aren't working through a permanent establishment in the UK, ie you don't have an office.
I hope this helps but let me know if you have any further questions.