The British used 51mm mortars in Flight/Platoon HQs until very recently (we had them with us when we deployed to Afghanistan in 2009), and they were primarily used to lob up illum. Originally the 40mm UGL was supposed to replace the 51mm, but due to the fact we still don't have 40mm illum in service, and because light 'commando' mortars are useful, they have been retained. In fact, the British forces recently introduced a 60mm replacement for the 51mm, which we also took with us to theatre when we went out. It's pretty much the same general design - barrel with integral baseplate, no bipod, simple sights. The Army infantry don't have a company level mortar at all, it's either platoon level (51mm or 60mm) or battalion level (81mm). RAF Regiment Squadrons are about the size of a reinforced company, and we do use 81mm mortars grouped into the fourth (Support Weapons) Flight.
During my tour, I was on the Support Weapons Flight of my squadron, and we spent about half our time firing 81mm mortars in support of the Rifle Flights (the other half was spent on patrols and OPs). In my experience, the 81mm is a very effective weapon system, though the weight of the weapon and ammunition means that manpacking it is not an easy proposition. We also mentored a Bulgarian unit stationed with us who were using 82mm mortars, as there were plans for them to take over some of our tasks and free us up for patrols, but their fire control procedures and very high failure rate meant it never came through (watching them fire illum one night, almost three quarters of the parachutes opened late, didn't open, or opened but then caught fire).
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