Anglian Regiment
-The L129A1 is a designated marksmans rifle in the British army in the same vein as the Howa DMR or Dragunov, I am not sure it is suitable as a primary rifle from an authenticity perspective.
-To my knowledge the M72A4 LAW was never procured by the MoD, suitable alternatives would be the ILAW (AT4) MBT LAW or maybe the Carl Gustav M2 (although I don't know when that weapon left service) you could give them the LAW80 but wheres the fun in that.
This unit is also a named infantry regiment which I am unsure is in-keeping with the general trend of generic unit names. An alternative to another UK Infantry line unit could be Airborne recon like the Para Pathfinders. A suitable loadout for them would be a C8/C7 rifle, ILAW and L129 (especially fitting as the Paras were first to be equipped with the new DMR) If you wanted to keep the Anglian regiment I would recommend a speed buff as they are light infantry irl.
Grenadier Guards
-Elite training might be giving the guards a bit too much credit, they're good soldiers (worthy of shock training) but not of the same standard as say SAS or Royal Marines.
-I don't think the MG5A1 machine gun is in general issue in the British army, if at all. the L110A2 (FN MINIMI) is a viable alternative although the Grenadiers, being heavy infantry, could also use the L7A3 which is still in widespread use.
SAS
-The SCAR-H is not officially in use by any branch of the UK military, if you want to give them a Battlerifle the SAS have been known to use a number G3 variants. Canadian C8/7 rifles are also popular as well as the M416 and M4.
Ghurkas
-MP7 was never adopted by the British Armed forces in a frontline combat role, a good close range alternative would be the L22 Carbine, a cut down L85A2.

-I have never heard of any British interest in the AMRAD program, what I do know is that the radars were removed and replaced with an optical-only aiming system cued by a remote radar. something to take into consideration.
-Additions to the UK's AA defences could include upgrades such as the Rapier FSB2, FSC or Rapier 2000. Another option would be the brand new FLAADS system just introduced to the British army as a Rapier replacement (CAMM L)
-An enhanced prototype howitzer could be added to the UK roster, the AS-90 'Braveheart'. longer 52cal gun = longer range and probably the ability to fire guided projectiles.

-The Scimitar could be upgraded to Scimitar II standard with improved armour, optics and FCS.
-Potential Marine recon helicopter, Lynx Wildcat armed with a battery of Lightweight Multirole Missiles (Lynx AH.7 would work as a model stand in sort of) http://u0v052dm9wl3gxo0y3lx0u44wz.wpengine.netdna-cdn.com/wp-content/uploads/2014/05/Wildcat-Helicopter-and-FASGW-L-Thales-Lightweight-Multirole-Missile-LMM.jpg

-What is a Challenger Brimstone and who told you about it? A more fitting alternative could be 'Exactor' a launch platform for Spike ER with a range of 20km, were used in Iraq.