I just bought a pair of Gelert reusable hand warmer gels from Amazon, for use when dealing with Fairy visitations.
I don't particularly suffer from cold hands on rides, a pair of bog standard gloves normally solves even the coldest of UK conditions, but you often need to take your gloves off to get tyres on and off, or for other more involved mechanicals. Even on the last FNRttC, when I had to sort out a quick repair on my rear wheel, after riding through a heap of glass a few minutes before, my hands were starting to be a little bit cold.
I've had a pair of chemical hand warmers in the past, but when we tried to trigger the reaction they did nothing, presumably the containing bags had sprung a slight leak, or they'd just become too old. This was on the Solstice ride a few years ago when neither I nor Wowbagger had gloves. I didn't mind too much, but Wowbagger was suffering a bit from it.
The Gelert ones are supposed to be reusable (you heat them up in a pan of water for twenty minutes), but it's worth an experiment at less than £4 for a pair! (Being Amazon, they initially offered me the order with first class postage, for an additional £6.
). The alternative option would be electrical ones, but faffing around with batteries and possibly separate units with leads, strikes me as being excessively complex if the Gelert ones will do the job.