I wouldn't know what's available in the UK but here in the states the shooting departments of our sporting goods stores stock cheap canvas belts with elastic loops intended for 12 gauge shotgun shells. The loops easily accommodate the plastic 35mm film canisters.

The same stores also stock accessories for the trap and skeet crowd that include some nice belt carried pouches - sort of like a fanny pack - but with a stiff opening that helps the shooter get out live rounds and drop spent ones with out looking or fumbling to open a flopping cloth pouch.

There's really a great deal of commonly useful gadgetry between the firearms shooting world and the photo shooting world.