Some ARs are exported to the UK. Most, if not all of these are either pump actions, or straight-pull bolt actions. Modifications typically include features that enhance operation in those modes, such as larger gripping surfaces for the charging handle, etc., which may conceivably be adaptable to semi-auto ARs. Whether or not they are an asset or liability in semi-auto mode is open for discussion.
I suggest you contact various Mfrs of ARs and ask if they export items to the UK, and if so, ask what they are and if they are available for sale in the US on a special order basis.
After all, no sense in re-inventing the wheel.
Some Brit shooting mags even offer Brit-made accessories for the AR as well as other US rifles exported to the UK, and such are available for int'l purchase.