I think you can safely and happily throw away all those cardboard 'Wads'...they have no place really in any Revolver Cartridges, unless you are making up Shot Shells.
There is no need to protect the base of the Bullet for any Black Powder era Revolver, or for any other Revolver either...that sort of thing is only for very high velocity Cartridges...and even then, it is not necessarily needed unless striving for the last word in accuracy over long distance.
You would do best to refer to Trailboss' own Loading Tables for .45 Colt in order to determine what range of Powder Charge for what Bullet weight ( assuming a usual Lead Bullet, which is the only kind your Webley really was meant for ).
I do not believe the Trailboss Propellant fills the Case as Black Powder would, so, there is going to be some 'Loading Density', or Air Space between Propellant and Bullet Base.
Bullet would seat to normal depth as per standard Loadings for whatever Bullet is in question.
I just went to Hodgdon's Web Site, and under "Hodgdon" Propellants, I factored in a 250 Grain Lead Bullet for .45 Colt, using "Trailboss", and it provides then the figures for various Loadings/Pressures/Performance with that Bullet.
Viz
http://www.hodgdonreloading.com/data/pistol
4 simple steps, just elect the appropriate check box for each step, and it gives you the loading Table for those factors.
Their max loading with a 250 Grain Lead Bullet, is rated to 12,700 PSI.
I epect the late 1800s Black Powder Loadings for the .455 Webley or the .45 Colt or for 44 or 45 anything Revolver, were often enough running around 13,000 to 14,000 PSI, ( particularly with some of the better "Sporting Powders' of the Day)... and,"SAAMI" for .45 Colt remains at 14,000 today...so, very friendly then.
"SAAMI" Chart
http://www.lasc.us/SAAMIMaxPressure.htm
'Trailboss' is widely celebrated and used, and is a mild and very friendly Propellant, and probably ideal for any of the old BP Cartidge Arms, when one wishes to load for them for pleasure Shooting with no worries about being hard on them.
Much easier clean up also, ( one swipe with a Bronze or even Nylon Bore Brush and some light Oil, then a cloth patch ) than Black Powder tends to be ( where one uses hot Soapy Water, dries all with a hand held Hair Drier, then Oils and puts away ).