“Sorry, annything over 50 Browning is under the NFA of 1934. That means the available weapons, such as the 20mm Lahti are highly controlled and registered with the goverment.”
In my country we need the same type of licence for a 22 rimfire as we would for one of these (seriously) and anything with a rifled barrel, unless classed as an antique (no licence required unless you choose to shoot it), or an airgun within a certain power limit,is registered with the government.
I purchased one of these a couple of months ago, I guess that you wouldn't be able to own one of these in the U.K.?
Bushmaster M4 A3