Sounds better than the 4.0 grains considered previously. Don't mind me reposting same answer as before, but actual reloading data for Mark VI be scarce.

From 1996 Alliant Reloaders guide for .455 Webley

Max loads with 265 HB RN L, CCI 300 primer

3.8 gns bullseye for 750 fps 12,600 cup
3.4 gns red dot for 685 fps 12,300 cup
3.6 gns green dot for 690 fps 12,400 cup
4.3 gns unique for 800 fps 12,600 cup
4.9 gns herco for 790 fps 12,700 cup
6.8 gns blue dot for 770 fps 12,600 cup

Have loaded for Webley Mark VI, and purposefully avoided bullseye and used lower charges of unique for less pressure more case fill. Remembering Alliant's loads are for hollow base bullets, and solid base bullets may give more pressure with same charge/col. Both of my Webleys (Mark 1 and Mark VI) have .449 cylinder throats.

Personally would prefer unique, and certainly not load close to max for a Mark IV. My memory ain't what it used to be, but seem to recall only the Mark V or Mark VI were designed for smokeless powder.