It does get hot in the UK, and we feel the effects more because we're not used to it.
Generally, we carry our water. You can either carry a bottle in your hand, or use a bottle belt, or use a "bladder" pack.
Some links to get you started as to what's available:
Runaid bottles
A UK stockist for Fuel belts
A UK stockist for Camelbak and other hydration systems.
Beware bottle belts with single large bottles, they tend to bounce.
I like the Fuel Belt. You can mix your refreshment - energy drink in some bottles, plain water in others. Or for when doing the housework, gin at the front, tonic at the back.