As bmibaby are not currently flying to Germany (thanks to part-owners LH I suspect), the only budget airline between the UK and Munich is easyJet to Stansted. If you're really on a budget, you could then take the train from Stansted to Bristol for around £30 return and it takes 3 hours. There's also easyJet from Berlin direct to Bristol if you can find some sort of cheap deal to SXF.
Cheers, Zac