yeah, not much in it, but the webley sights are much better, and it's much easier to find spares for. That said I much prefer the Britiania design, and the weight distribution makes it feel much lighter in the shoulder.
It's probably no harder to find a .177 Britannia than it is to find a .177 Service, if the calibre matters, but .22 Services are everywhere. The BSA I'd say is indeed probably the best of the three; accurate, robust, and as said above, cheaper and very easy to work on.
In terms of shooting, all three are a bit slammy as standard. It's very easy to sleeve the transfer port down on the britannia and the BSA, making them noticably softer to shoot (and adding a little power), but the Webley you can't do much with, although an o-ring piston seal helps. I should say that they are all better in .177 in this respect, especially the BSA.
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.