Rapid white is good, i bought one on ebay for a tenner, glow white i think it was, ill double check if your interested, was a bit unsure when i purchased it but its good...a wee bit tricky and uncomfy, gum shields you put this jelly stuff in, but it works, you do it everyday for about a week...it does have bleach in though!! Bleach is good...white :-)
The best thing you can buy are Crest White Strips. Don't know if you can buy them in this country in the shops, if not you can get them on the net, amazon etc. They are easy to use and really work. All my friends got them after they saw my results.
I recently bought a tube of 'Arm & Hammer Enamel Care' for natural whitening. It reads 'restores surface enamel, with liquid calcium - but I don't suppose it'll be much different from any others I've tried.