The films were predominantly shot at Elstree Studios in London. Exceptions being the exterior shots (Tatooine - Tunisia; Hoth - Norway; Endor - Redwood; plus a few pickups in Arizona) of course. So most of the 'extra' actors are British, including Wedge, Biggs, Red Leader Garvin Dreis and the other fighter pilots and Rebel soldiers are British. Many of them had their voices dubbed in ADR.
David Prowse and Peter Mayhew were also Englishmen who wore the costumes for Vader and Chewy respectively, provided placeholder voices - yes, even Chewy - which was obviously replaced later. Greedo was the same.