I can reccomend JD airsoft on that map, only dealt with them a few times & only bought ammo but I had some work done on my gun there. They got the job done quickly and I got my gun back without hassle & (touch wood) it hasn't broken yet.
If you can get a train from Birmingham to Berkswell/Balsall Common, there's Gunman Midlands a short (twenty minute) walk from the station (can be found by looking for directions on google maps, you want the grange, cv7 7fp), they're open 7 days.