You live in the UK, so you CAN have a .50cal BMG rifle, just like the ~130 other .50cal shooters here. They are members of the Fifty Calibre Shooting Association of GB - google them to learn all about it.
Having a FAC already is a good step, though, as it implies that you are already knowledgeable about live Section 1 firearms.
You just need to be fairly well off, though - one of the top-class UK shooters sends between 1500 and 1700 rounds downrange every year in practice - at £17.50 per shot. Realoding equipment is a mite costy, too - figure on spending around a grand to get going. Each shot likely costs around a £1.25 in powder alone, and each of his four rifles cost him around £10K, too.
tac