Deet is the the active ingredient that works best.
If you can get a product that is more like a hand lotion consistency than like a liquid spray, you'll find it to work best as it'll stay on you longer and not leave that oily feeling.
I usually go to REI to buy such products. If you can find a good outdoorsy store, I'm sure you'll find a similar product there in the UK.
This is the stuff I use:
http://www.rei.com/product/686229