Before shooting, I suggest you slug the bore & look closely at the marks left by the grooves on the slug. There were no real standards back then, particularly with something like a Purdey. Each big-name maker had their own ideas & even the patterns of rifling varied. If you know the groove diameter and the general shape of the lands & grooves, you can better select the proper bullet. You might also consider making a cast of the chamber , throat and the first inch or so of the barrel with something Cerrosafe. The rifle is probably chambered for the British military case, but there were other .303s that used quite different cases.
The 1:10 twist would stabilize almost any bullet you select. Looking at the shelves in my garage, I find Hornady & Sierra bullets with box labels indicating diameters of .310, .311, .312 & .313. Bullets like those are probably easier to find (and will be less costly) than Woodleighs.
I have an 1893 Martini/Enfield converted to .303 (military) in 1897. My rifle shoots well with .311" dia. Hornady 174 or 180 grain round-nose bullets and about 36 grains of IMR-3031. Any of the IMR powders with moderate burn rates should work (3031, 4064, 4895, 4320). Try to find at least 2 different loading handbooks from the 1940s or 50s.
