Teeth whitening kits?

Do they actually work? I want to get my teeth really white but don't wanna pay for the proper one which costs more than £200. I've seen teeth whitening kits in Boots for about £15. Anyone who has used- them do they work well? Thank you
--------------------
The cheap ones have the same ingredients and they work just fine. Just buy the generic brand. Leave it in as long as possible but don't fall asleep with it in, trust me that sucks. Also you might want to use the Listerine Whitening Mouthwash, that works great if you use it twice a day. And its under ten bucks.
Source