Well England isn't known for its heat but on trips across Europe I've ridden in high temperatures a bit and I used to race. Heaving a bike round a track can get you sweating a bit, and a full face helmet soaks it all up.
Some high-end helmets have removable liners that can be removed and washed (most moto-cross helmets do as well. If you don't have the joy of a removable liner you can wash your helmet. They are desinged to be worn in the heaviest of rain so you aren't going to damage it (but don't put it in the tumble-dryer).
I've washed mine in a shower. Get it good and wet and then use baby shampoo or other MILD detergent (shower gel is probably as good and smells more manly). You won't need a lot to work up a lather and rub it into all areas. Then made sure you rinse very well. Several rinses will be required and squeeze the lather out of all of the pads while spraying gently with warm.
Your helmet will now be clean and fresh, but very wet and heavier than normal and slippery. Don't drop it at this point. Push a towel into the inside to soak as much water out of the padding and then remove it.
Now you need somewhere warm and dry to dry it out. I use the airing cupboard. Remember to stand it on a towel as quite a lot of water may come out for a while.
It will probably take 3 days or so to dry properly.
Don't be tempted to use a hair dryer of other device to dry it out quicker. This can damage the padding and foam liner.
You can buy dedicated helmet claening and deodourising products in motorcycle clothing stores. I've never used them.
