The first imported Olympic Arms rifles into the UK had either a gas port or a gas tube fitted, but not both
I owned one with the gas tube fitted, nothing illegal until the two go together.
Otherwise a barrel drilled compensator would be illegal..........
The imported rifles, all got batch tested by the Home Office and eventually returned to thier owners in the condition they were imported. Well nearly all, I think a gas key carrier got welded over on one
Since then, I think, every 'new company' marked AR has been pulled for forensic checks!
Those that I know of that are approved are :-
OLYMPIC ARMS
BUSHMASTER
PWA
DPMS
ASA
OBERLAND ARMS
COLT*
(*I know someone who has a US Gov Property marked Colt M16A2 lower ,that was passed Section 1 legal in the UK by the Police / Home Office........
)