[Real steel 249 para muzzle device and blank firing adapter]

Yep, you definitely need a slot on a section 1 FAC for a flash hider (comes in the same category as a sound moderator) s.57c of the firearms act if you want to look it up.

 

This is just a related post of someone else who queried it with the home office for your reference.

https://www.stegough.com/flash-hiders-and-flash-suppressors-in-the-uk/

[WTB) A_TRD LVAW Handguard Legacy]

It might be worth contacting Killen Airsoft on IG... He's doing another run of them although he's based in Sweden. People seem to be happy with his work.