Ah one of the few parts of firearms legislation where the UK is actually better off than the US, there aren't many!!
Considering how much hassle you guys have to go through for one and as they seem to be more expensive than over here, i wouldn't mess around with a rimfire only suppressor, i'd get something that will work with a .223 AR as well. Something with a quick-detach mount so you can easily switch it between your 15-22 and 'proper' AR.
As for reliability, with it being a straight blow-back gun it
should either have no effect or actually make it cycle better with sub-sonic