The original plastic sight was the same general shape, but was relieved on the bottom of the ramp to allow for the Osprey's thicker barrel. The original sights aren't interchangeable. However, Webley re-designed their fore-sights part way through the Hawk MkIII production run so that they were made of alloy and much stronger than the originals, which were prone to cracking. The alloy sights are interchangeable between Hawk, Osprey and Vulcan Mk1. They may fit others, but I'm not sure. The ones Chambers will be supplying are the later, alloy ones.
The South of England has 2 good things, the M1 and the A1. Both will take you to Yorkshire.