I was thinking I might go for an SMG again - they're one of my personal favourite kinds of weapons, and my airsoft G36 put me off of them a bit in general (though I know the real one is more capable). Maybe an MP5 again, as I'm fond of them. A UMP wouldn't be too bad either, though I'm not aware of the SAS using them to any extent. 
Sidearm wise, I'll stick with a Mk.23 as well, and whatever survival/combat knife I had before, and a suitable selection of grenades. Probably no need for AT this time, but I'll pack a LAW if possible, 'just in case'.