In general terms, if you take your gross pay (all allowances, Flying Pay, etc.) and pay normal tax, your annual take-home will be very similar to the advertised basic pay, if you see what I mean. For example:
A year 1 FO gets £42000 ish basic and about £12000 in allowances on top. By the time you pay tax, pension contributions, etc, you will take home approx £42000 per year or about £3500 per month.
As far as being German in BA, the only relevance is whether or not you intend to commute from Germany. There is (at least) one German co-pilot on the 777.