Debenhams is the best by far, because it brings together big brands and makes them cheap(ish).
For a good quality Maxi dress for Β£60-100, go for something like Coast, but buy through Debenhams (in their shop they go up to much more than that). This is (sadly) a normal price range for the lower end of decent formal evening-wear. Men might expect to spend this much or more on a cheap Tuxedo, and people can spend a lot more.
It's kind of pot luck what you get in the store (though big stores will have a 'reduced' rail and you can get some very good things). If you buy online, be careful to check the material because some things can look very good on a model in the picture, but look quite bad in real life.
If it's just a school prom, you can get something cheaper, but Debenhams is still good for high quality, low price.