Freewheel not flywheel, and I can't begin to explain it as there are at least four possible ways you might have to do it depending on the type and model of the hub.

As for the BB, there's a multitude of different BB shells your frame could have (BMX Mid, BMX Spanish, Ashtabula/BMX USA, threaded/BMX euro and probably more I can't think of) with various different degrees of replaceability. For instance, if you have a BMX Mid BB, with BMX-type cranks, all you need to replace is the bearings. If you have a threaded BB shell and a cartridge BB, you'll have to replace the spindle as well as the bearings. There are one hell of a lot of potential combinations.

Where in the UK are you located?