I also have an early-build Milbro 27 with two-stage trigger. Just my opinion, but I suspect the trigger parts are actually pre-war German ones that Milbro "inherited" with their purchase of Diana machinery after the war (the sights, and many other smaller parts on that gun, are identical to a pre-war German 27 that I have). The Milbro guns got different trigger designs when these parts were gone.
I don't have a two-stage Webley Mk 3, but I would assume those trigger parts were new-made by Webley, and would guess they don't interchange with the 27...but that's only a guess!