Yep, the 15-22 in the UK is exactly the same as that in the US, so all (factory threaded barrels) will have 1/2 x 28 threads (UNEF). Indeed, there are no restrictions on the 15-22 in the UK (no 'compliant' models required here), so threaded barrels, flash hiders, large mags, collapsible stocks are all good to go

On the UK FAC questions:

Firearms and ammunition that is listed on an FAC can be kept at the licence holder's home, but must be kept securely - generally this requires suitably securely fixed separate gun and ammo safes. This has to satisfy Home Office guidelines and local police firearms licensing officers, so wall racks and glass cabinets would not really pass muster!

The FAC will also stipulate where the firearms can be used (e.g. mine is for target shooting, so that includes my own range, any that are affiliated to that, and any upon which the appropriate insurance is in place for me to shoot). For over land shooting, the licence holder would require permission of the land owner and the land would have to be considered safe to shoot on - there are restrictions relating to using a firearm near to a public highway, for example.

Obtaining an FAC is not particularly hard, as long as you are deemed fit to hold one and can demonstrate good reason for needing particular firearms (e.g. sport shooting of a relevant discipline, or use for hunting/pest control). Application is to the local police firearms licensing team and requires filling out a form relating to your personal details, medical state (also allowing the police to approach your doctor for info) and disclosure of any previous offences (if applicable). Also required are 2 character references (including one from a club official if a member of such - in order to hold an FAC for target shooting, you need to be a member of a Home Office approved shooting club).

FAC applications currently cost £50. Granted FACs last for 5 years, with renewals costing £40. As an FAC will stipulate exactly what types and calibres of firearms can be purchased, official variation of these is often required to accommodate the licence holder's change of intention as to what firearms he/she wants to acquire. A "one-for-one" variation (simply changing one slot entry for another) is free, but adding any slots would cost £26.

Whilst there is technically no specific limit on the number of firearms you can hold on an FAC, the granting of these is at the approval of the local licensing team. As such, first applications are unlikely to be allowed for a large cache of guns (licensing officers dislike speculative 'shopping lists' on FACs!). Also, many authorities stipulate that a home alarm system with telephone call out is required for possession and storage of 10+ firearms. I'm at 9 slots, and that'll do me fine for a while! On topic: each moderator/suppressor held would also require an authorised FAC slot.

There are many more subtleties to the UK licensing system (each separate county police authority has a different 'take' on the overarching Firearms Act and Home Office guidance), but in a nutshell, that's about it