I, too, keep bees here in the U.S. I've become quite resistant to bee sting effects over the years, but I've noticed the odd Africanized bee or two hanging around the hive yard. Nothing serious yet, but enough to be only slightly alarmed. Have you seen any unusual bees or hasn't the African bee threat invaded the U.K. yet?
These bees are quite a bit more energetic and antagonisitc than our Italian hive bees. Reports indicate the stings are more severe in toxin content (as well as aggresiveness).
I keep a smoker going when I'm working the hives and, if stung, I immediately, eject several puffs of smoke in the area. This tends to lessen the warning alert given by a bee that's just stung someone (and is dying) keeping the others calmer.
Be sure to not become overly frightened while in the bee yard. They sense anxiety (probably pheremones) and makes them more readily available for stinging.
I suspect the sting on your wrist happened to be in an area the allowed the venom to quickly migrate up your arm... more so than other areas... but, admittedly, that's just a guess. At any rate... don't give up!
Best of luck!