The interest is a little better (say 2% better), although I see no reason why you cannot get an offshore savings account in Sterling. At the end of the day you have to pay German tax on your income anyway - they are not interested (excuse pun) in savings you might make with an ISA. So for the German tax authorities a UK ISA is the same as an offshore savings account...if tax is due in Germany then you pay tax in Germany. Germany taxes you on your worldwide income.
To compare offshore savings rates go here:
http://www.moneysupermarket.com/As they are OFFSHORE accounts I do not envisage a problem that you are in Germany!!
**** HOWEVER *****
The big issue is that you are taking a foreign exchange risk. If we assume you are later gonna want your money in €uros then if things go against you (the pound gets weaker against the €uro) then this could easily wipe out the extra 2% interest you are earning. However the foreign exchange could actually go in your favour, and personally I like to have some money in €uros and some in Sterling for this reason.