It was not clear if the OP was looking for feedback on the use of 16 ga slugs or was looking to buy 16 ga slugs in the UK. I've shot a fair number of 16 ga slug 25+ years ago in the UK for practical shotgun shooting and more recently outside the UK for boar and deer hunting. Typically my 16 ga use has been limited to the simpler lead type slugs and has not included any sabot type slug ammunition. I own a number of 16 ga drillings and combination guns, my favorite being a Blaser BBF97 over under that I fitted with a small red dot sight and sometimes a low power scope.
I've shot 12 ga slugs out to 300 yards, and had several 12 ga rifled barrels for pumps or semi autos on my UK FAC for practical shotgun use. For 16 ga slugs it has always been smooth barrel shooting for me, accuracy was reasonable as drillings and combo guns typically have either rifle sights or scopes, this helped a lot even if the sights were zeroed to the rifle barrel, with a bit of practice you can work out where the slug would go in comparison to the rifle barrel. On my Blaser BBF97 I zeroed both barrels to the red dot sight as the barrels are adjustable to each other as they are on several Valmet combination guns that I own.
As far as hunting performance went, anything I ever hit with a 16 ga slug certainly fell over and stayed down. Out here in Michigan the lower half of the state was shotgun only up until recently when 2" straight wall rifle calibers were approved, there was a lot of deer shot with slug or buckshot for many decades. I used to shoot quite a few deer with a scoped bolt action slug rifle in 12 ga, these worked very well with sabot slugs (Remington Copper Slugs). On occasions I would shoot one of my drillings with the two shotgun barrels loaded with 16 ga slugs and no rifle round as the use of rifle ammunition was banned in the lower more populated half of the state. It was just like shooting a large caliber double rifle. In Europe the 16 ga remains very popular, I have shot boar with an over under combination gun, slug on top and rifle barrel below.
I still have a stash of 16 ga slug & buckshot in my US basement. If you have a legal UK use for it and have it on your FAC I don't see any reason why you could not track down some 16 ga slugs in the UK or if you travel buy some and bring them back to the UK with you, of course declaring them on entry. I used to bring a lot back from the US to the UK in the 80's and 90's in every caliber.
I need to get back to shooting a few more slugs!
