Airfix "Guards Band" and "Guards Colour Party" may give you all the marching musician and marching figures + 2 Guard Houses in nice Bearskin Hats and removable accessory pieces you'll need. You can find them in deep red colour on eBay.
If you're looking to have some action figures, I liked best the Zulu Wars British Infantry and Officers sets made by another company. Myself not fancying painting, I found them a good colour match for doing "head swops". I think they look nice with the front two ammo pouches on them too plus you get horse officer with arm swops.
The Airfix sets, I think are majorly underrated. Not the best, but certainly look good all set up. I've got both sets in red. You can typically find some reasonably prices "Loose; no box" so don't overpay. And they do look cool with head swops as action figures with other Zulu Wars 24th.
And if you really want to jazz-up your London Diorama, you should add a marching group of Strelets Nappy Highlanders marching. You can swop onto some the big drum. You'll get very close to those favorites that Britains and Crescent made decades ago in 54mm.
You can check out all these figures on Plastic Soldier Review under Manufacturers button/link for great scans (TX PSR)!
Regarless of what figures you use, please tell us when you post your work to your site, Mike! I've always liked your Christmas set-ups.
these sets got me hooked on 20mm scale!!! 6 decades later still collecting!! the plastic colour of these Guards sets when i bought them in 1960 (for pennies) was a pink colour!!!! and Airfix magazine and annuals provided great ideas for conversions, there was series on Napoleonics, ACW, and other periods as well as ideas for converting Airfix railway buildings and forts
stay safe out there
cheers Old John