Simpson depicts a French Naval battery at Sebastopol. The cannon looks like a straightforward conversion of the Strelets Russian Naval Cannon depending on how fussy you are. Going back to Nappy artillery for horse artillery is good advice but forward to the American Civil War for mortars, gabions and figs for the siege. A need for heavy stuff (position artillery- RA) 18,32 pdrs,8in howitzers, etc but probably needed for ACW and Nappy as well, so some manufacturer ought to have them on their "to do" list.
I would not see the Emhar figures as competition but complementary. Sometimes one manufacturer makes up for a deficiency in another's figs eg standard poses as opposed to a wide but florid variety, back packs or no back packs and so on.
Re: some ideas for British Seige Crimean artillery?
The Russian made set by LW #39 Medieval Mortar makes the perfect British Seige Mortar by simple conversion.
Use only 2 of the pieces, the base and barrel. Simply (1) cut the "T" shape barrel mount off the bottom of the barrel, (2) drill a hole through the bottom of the barrel and plug only the straight portion of the T back into the bottom of the barrel, and lastly (3) press fit it onto the base backwards facing at an angle and, presto! Looks close enough for me to the mortars in the Osprey "Balaclava 1854" book pp. 32 & 34. Perfect mate (and opposition) to the Strelets*R Russian cannons. That easy