The big sparkly taffeta dresses that you come across when literally typing into Google 'prom dresses' are unrealistic in many, many ways and if you ask me, they really aren't too flattering either. While you may think unless you get a big fancy dress you won't look good, you're wrong. Bigger does not equal better and most girls will be avoiding dresses along the 'traditional prom' line anyway. You can get plenty of flattering plain dresses and then use accessories to make it a little fancier! e.g. a skater dress and an embellished collar...
or maybe combine a skirt and top instead of going for the typical dress?
The great thing about these ideas is that the items you buy will probably be wearable for more than just the one occasion, making it a much more worthwhile investment.
Or try a different idea all together and rent out a dress for the day/night for under £100 with girlmeetsdress.com. Their dresses really are something special (red carpet material) so I'd certainly recommend taking a look at their website. The only downfall to this trick is that you'd have to be on the look out for potential spillages at all times!
And finally, if you are still stuck, head to shopstyle.co.uk. Simply type in 'dresses' into the search bar and just like that you are presented with 1139 pages of dresses to choose from, from all kinds of different shops, for all occasions and at all prices.