Nobody uses the SLR anymore; the SA-80 isn't great for parades, but it's okay.
There are still a few SLR's laying around in armouries, but they are not officially used.
Remember, those guards are doing a real job, they are not just there for the ceremonial side of things, hence why in that picture they still have the SUSAT.
Ferrer, the SAS use numerous weapons, including the SA-80, the C7, C8, M203, Minimi, Gimpy, G3, MP5 etc etc....
The C7 allows for greater deniability of operations than the SA-80, and the regiment were using the M16 long before the SA-80 was on the scene. Not that the M16 doesnt have some advantages, despite its awful operating system. It is lighter and easier to wield and march with for a start.