Good news on the date- I'm finding you get lulls and then a few at once at the moment!
Sounds like you are best out of the work situation- it's funny how quick it can turn on office politics and no one is immune from Machiavellian dealings. Been there done that- don't know anyone who hasn't and suffice to say usually works out significantly better.
It's difficult to answer your question - google London weighting for some of the big corporations to get an idea, rule of thumb used to be about 7k.
Other way is work back from what you need to survive here- even zone 1 to 4 Oyster can be 200 quid a month etc. etc. and then add on a premium that you'll be happy with.
Never been Berlin but it's on my list - heard nothing but good things.
Exciting times ahead whatever you do.