I have a HB
for my Sentria, and you are right. It is air powered now. Unfortunately, it has been reduced to be more of a Dremel tool, rather than the full sized tool it was in the past. It's amazing how much the Handi Butler changed from the Classic Omega through the Heritage.
Omega Butler (and all previous models) Grinding wheel and knife sharpener, as well as the drill, buffing pads, wire wheel, disc sander, and lambs wool buffer (fit over disk sander disk for buffing anything from cars to hard wood furniture).
Classic III Butler: Added jig saw feature and removed the grinder/knife sharpener. All other features of previous Butlers.
Tradition Butler: Same as Classic III Butler except that mid run, the jig saw was removed. Safety issues. All other features retained (except for grinding wheel/knife sharpener).
Heritage Butler: Drill, buffing wheels, wire wheel, and disc sander, and lambs wool buffing pad. Discontinued when Turbo Group was released.
Sentria: Handi Butler reintroduced. Worked off of air instead of belt driven attachment. Miniature tools like on Dremel tool. Didn't last long and I believe it has been discontinued again.
It's rare, and I don't know if it was sold in the UK, but I see Heritage HBs come up once in a great while. All you would really need is the power transfer unit that connects to the Kirby. Everything else from the Tradition HB would then work off of that. I can't make any promises, but if you like, I'll keep my eyes open and let you know if I find one.
I used to have a 500 series power plant on my workbench fitted with a HB and used it exclusively for polishing Kirby and Royal vacuums. It may have taken longer than it would have with a professional grade buffer, but with patience, I got some pretty good results, (although, I don't think the Royals took too kindly to being at the mercy of the Kirby

).