I'm pretty sure Liege was a source for barrel tubes, to be later proofed in Britain.

British makers bought Belgian actions such as the Bastin action with sliding barrels (Google Purdey Bastin slide action for a fine example), to be made up in Britain. The Bastin action was used by a number of British makers, and it was one of the first non-Lefaucheux pinfire actions in Britain (a Francotte with Bastin action was used in the Trial of 1859). Some makers brought in barrelled actions (or possibly whole guns) from liege as well, with a British maker's name on the rib and Belgian action and barrel proofs as evidence. And some makers operated in both London and Liege, such as the Masu Brothers.