That's the catch 22- you find it difficult to keep hydrated because of nausea and sickness, both of which can be caused by dehydration. You can get ORS tablets/powders that dissolve in water. They provide immediate rehydration. In the US the brand is DripDrop (which you can get on amazon UK). If you can stomach coconut water, that you can get anywhere. I don't like the stuff, but many do. Another thing is squeezing lemon juice into your water. I find that helps, but then I'm only dealing with nausea and not vomiting.
You can always ask about which electrolyte supplements are best. I have used the sport ones, but not all would be suitable. I never get ones with caffeine, but some do have that added.
Hope you find some relief soon.