If you are living in Germany you must have a German bank. That's not a legal requirement, just a fact of life. All banks are unique to their country.For example , if there is a Barclays in Germany it will be legally separate from British Barclays, governed by German banking laws, a German company (albeit part of Barclays' group) and as German as any other bank there. You can't treat a branch in Germany as a 'branch' of the English bank.
Living in Germany you'll need to set up standing orders for uitilities, mobile phone etc and have a debit card and so on ,and pay all the ordinary bills of life, and that can only be done with a German bank. Any German income should go into a German bank. You can't live on the basis that everytime you want to pay a bill or draw cash from a machine you have to go through foreign exchange , with charges !
There's nothing to stop you keeping an account in an English bank in England. In fact it may be convenient to do so.You may have people or firms in England who want paying in sterling in England, so keeping some money in a live account there may be useful.