Pop in to your local Motorrad dealer with a nice smile on your face, wearing the jacket, and ask them if they can recommend anything.
Chances are the chap on the desk will reach under the counter and pull out a BMW zip repair kit.....toss it across to you and job's a good'un.....free of charge.
That's what happened to me anyway, with my similarly out of warranty streetguard jacket, at the sadly missed Prestige BumW dealers in Bexleyheath
Failing that, any decent seamstress should be able to do a pukka job on it....go for the oldest, most nanny looking haberdasher's in the town and again, approach with your most charming smile......it'll probably cost you under a tenner
EDIT....I see you're across the water there.....you may find that the 6 web fingered types over the border might not be able to do a good job on new fangled idea like a zip......get the ferry across to 'Arrich here and I'll show you where it can be done properly