Rhingyll,
I think that's a tough one using just what's available for purchase right now.
Wargames Foundry has one "walking" figure on foot in their Zulu War range "British Command and Characters" pack, dressed in shirtsleeves. But he's only one out of 6 figures in the pack, plus he has a shirt on instead of a jacket, and he's an old, smaller 28mm figure, so maybe not a great choice.
If I absolutely had to cobble together some 28mm "walking" Royal Artillery figures tomorrow… I think I'd buy a pack of Perry Sudan range Egyptian or Sudanese Infantry marching, a pack of Egyptian command marching, and a pack of Perry British infantry for their helmeted heads.
The Egyptians all wear minimal belt equipment which could easily pass for Royal Artillery gunners (you might trim off the ammo pouch on the back of their belts if you want).
The marching command pack has 4 figures with their left arms at their sides which I believe could be cut off and used to replace the left arms carrying shouldered rifles on the marching Infantry figures without too much difficulty.
Swap the Egyptian fez-wearing heads out for your choice of British head -- maybe the cloth-covered version that comes on the "Indian service dress" figures, so they fit even better in Afghanistan -- and you are done. More than a little work, but not a terribly large amount, especially if you only need a few of your "walking" artillery crew conversions.
They will be wearing leggings but these can be carved into puttees without too much trouble, or just left as is and painted to look like puttees.
Besides the conversion work, the painful part is you would need parts from 2 additional "donor" figures for every finished figure, so they will cost three times as much. The good part is that all the components were sculpted by the same person, so they won't end up looking like Frankenstein monsters.
Unfortunately I think all the figures in both the Egyptian and Sudanese marching infantry packs are marching in step with their left foot forward, so you would not get your right foot forward variation.