Be careful, people. I called my husband's UK dentist when I was still in the US to check the price, and they are totally DIFFERENT. The cheap ones(under £100) are not the hard fit-to-your-mouth kind you get in the US for $4-500. They are generic soft plastic guards, according to his dentist. My dentist in the US(who is also a friend) said those are not good because everyone's grinding pattern is different. Plus, the soft ones do not keep your jaw from moving as well as the hard plastic ones. They can actually harm your teeth. I, personally, would not take a chance on a cheap one, because you won't know the damage until it's too late. You may end up spending more in the long run.
Please check with your individual dentists and make sure you are comparing apples to apples.