It will be legal providing, of course,you have 'good reason'. Target shooting is a valid reason to have a .50 precision rifle, so a .408 should be no different.
I shoot with the FCSA and they use several military field firing ranges in the UK.
Diggle ranges in Oldham, have 100-1000 yards facilities, so it could be used there too.
The ammo for a .408, If I recall correctly is very expensive.
I was thinking something along those lines otherwise as you say there wouldnt be a 50 club at all.
I was trying to get a feel of what the powers that be use to rationalise granting of FAC or does that vary county by county