I am afraid the SAS will not have switched to a SIG rifle, the only weapon they will have switched to is the HK 53 (MP5 in 5.56mm).
To my knowledge they are still using the Pigskin waistcoats as the carry all ammunition on their leg, the vest caries 4x Flashbangs or CS grenades, an axe, a knife, and baton rounds for the breaching shotgun.
Modern look
PASGT (looks like the Bristol made helmet they use)
S10(with airsoft safe lenses)
Pigskin or Black MOLLE vest with Grenade pouches
Black overalls (they moved off the blue a while ago)
Hip holster for the pistol (they don't use Drop legs for pistols as they aren't as secure)
Drop leg for the SMG/Rifle mags
The SIG 552 is really not going to do the loadout any favours, stick with an MP5 or people will just tell you how bad it looks. At the moment you look will be a SWAT loadout not SAS CRW.