Real Surefire Flash Hiders are ILLEGAL unless subject to conditions on a Firearms Certificate with the correct variation ergo they are ILLEGAL on Airsoft Weapons.
See the bold text
Section 57 Firearms Act -
In this Act, the expression â€œfirearmâ€ means a lethal barrelled weapon of any description from which any shot, bullet or other missile can be discharged and includesâ€”
a - any prohibited weapon, whether it is such a lethal weapon as aforesaid or not; and
b - any component part of such a lethal or prohibited weapon; and
c - any accessory to any such weapon designed or adapted to diminish the noise or flash caused by firing the weapon;
Sales of any part of a firearm which contravenes the Firearms Act will not be permitted.
A real steel upper, even if supposedly modified, does NOT change it's original classification - it falls under the classification of "component part" (from the judgement of R v Clarke (F), 82 Cr App R 308. Ergo, it's either S1 or S5 (depending on how bloody minded the CPS are). This is why Section 1 full-bore straight-pull AR's built to UK Section 1 spec have to be manufactured as straight pull from the ground up!