Royal electrical mechanical engineers stable belt. There seems to be several schools of thought as to how the stable belt came about here are two of them. The modern 'stable belt' owes it's origins to the belt, woollen, a money belt type item issued to all soldiers, not just cavalry. This was a wide belt made of light webbing with the same leather strap arrangement, and a leather pocket. These were very often decorated with badges and other personal affectations, and were also available commercially with embroidered coloured designs.
And a second school of thought. The stable belt originated from the practice of the soldiery removing jackets and using canvas straps as belts when performing tasks in the stables. At the turn of the century the Army was still largely horse borne and it was not just the cavalry who had horses. The strap belt was adapted and at some time produced in regimental colours which caught on and today all regiment have their own design.

Thanks to Mel Dundas-Taylor for this third suggestion . I believe the origin of the stable belt is simpler than you suggest it is the cinch strap which goes under the horse to hold the saddle. The mounted soldier would remove the strap and keep it thus making the saddle unusable. Have a look at modern cinch straps at your local pony club they are almost identical.

Anklets web or gaiters dated 1943. Worn when wearing battledress. There were a lest three different patterns of infantry gaiters used during WW11 the ones shown are the third pattern. The first pattern has web straps with brass tips in the second pattern the brass tips were deleted for economy reasons. The third pattern has leather straps and internal leather reinforcements at the bottom which were first used in 1941. As you can see on the picture these belonged to a Mr R.A.Barber.