Raynaud's sufferer, what works for me:
- Gloves gloves gloves. They don't have to be amazing but you need them to be dry. Minimum I wear liner gloves + something else and then take 3 sets with me (set of liners, 2 proper sets of gloves), if it's likely an all day Scottish winter trip I take between 4 and 6 sets (2 liners, 2-3 standard sets, 1-2 set of super warm mittens).
- Get some heat packs you can slip inside your gloves. The ones with special places to put these are useful but you don't need them, just use a thin pair of liner gloves.
- Keep well hydrated
- Time release Niacin tablets (which acts as a vasodilator)
- Don't bring coffee in your flask, try a hot sugary drink (hot chocolate, hot ribena, whatever you like) since caffeine doesn't help
- Keep your core warm, which keeps blood flow to hands and feet from shutting off if you catch a chill. Annoying in normal people but for people who have vaso constriction issues this can stop my hands working entirely
- Keep some snacks handy to keep your blood sugar consistent
If none of this works and you have serious need for your hands to work properly, for example you're into winter climbing where hands 100% need to perform or it's a safety issue, then I'd consider seeing an allergies specialist who can help you with figuring out if it's a diet issue which acts as a catalyst for your poor blood flow by causing inflammation. About £300 or thereabouts for a reasonable run up.