The Webley Mk II came out in 1895 - the .455 cordite loading came out in 1897 (IAW Wikipedia) - which means your Webley was made for black powder. Proceed with extreme caution if you're really going to use smokeless in it...
I've not used Unique or Blue Dot in my Mk VI; I used Unique in my .45 ACP, 3.7 gr was pretty variable for velocity (548 - 617 fps) and a bit smoky, but accurate. 4.0 gr Unique gave me 5 shots, 4 of them within 10 fps of each other and all under 600 fps, this may be a good load. The Mk VI loves 255gr lead bullets; LSWC is all I load in .455 brass.
The powder I used most for the Mk VI is Trail Boss, which it says on the bottle was formulated for low-velocity lead loads; 3.2 gr with a 255gr LSWC gave ~600 fps (remembering my barrel is 2" longer, they should be slower in yours). I went as low as 2.9 gr Trail Boss, but I
do not recommend using that little powder - I had extreme variability of velocity (371 - 598 fps) with that load. Trail Boss is also dirty and smoky, that's a problem you'll be stuck with because the flash gap bleeds pressure so it's not easy to get all the powder to ignite.
My primo-ultra-accuracy .45 load is 3.6 gr Vihtavuori N310, loaded in .45 ACP cases with 200gr LSWC's (I have both the original .455 cylinder and a spare cut-down cylinder for .45 ACP / moon clips) but this is likely dangerously warm for your Mk II. I shot this load in the Mk VI and it left powder granules in the cylinder - I'd have to recommend staying away from VV N310, again, not enough pressure to ignite it all.
- and the bad news? - it's not an "antique", because ammunition is still commercially available for it...
