Definitely dehydration.
I generally drink 500ml an hour, although I realised towards the end of today's ride that I'd barely had half that, and have been making a point of rehydrating since I've been home.
I'd recommend doubling the amount you drink during your ride, if that's all you can face, then drink 500ml every hour when you get back until your wee is straw coloured. It doesn't have to be water. Squash, fizzy drinks, even tea or coffee will help - the fluid in tea and coffee outweighs the dehydrating effect from caffeine being a diuretic. If you're eating plenty, I can't imagine you'd need to add electrolytes to your water at this time of year. I don't (although I use them religiously in summer), and it's still a lot warmer here than in the UK.