If the broad arrow on the top strap of a .455 is rather big then it's a Royal Navy mark. These big broad arrow marks often look hand done. To confirm Royal Navy look above the back grip strap on the frame for the letter "N" This is known to be a Royal Navy ownership mark. The rest of the revolver will have various British Govt. inspection marks and small broad arrow marks on the various parts. These little marks are found on all Govt. issue Webleys.
As noted above by Jack, their are lots of other marks which can help you trace the ownership of arms used in various parts of the Empire, and sometimes by individual units. These things make collecting such weapons extra fun and interesting.