This is probably not the most ethical, but there are some sources claiming that this is indeed possible to do...
http://www.appleinsider.com/articles/09/08/27/apples_snow_leopard_disc_will_install_on_tiger_macs.html
http://theappleblog.com/2009/08/27/tip-from-mossberg-snow-leopard-upgrade-good-for-all/
http://lifehacker.com/5347086/confirmed-29-snow-leopard-installs-whether-or-not-youve-got-leopard
However, you must have an Intel Mac (Snow Leopard doesn't work on PowerPC Macs). Also note that some older applications/software won't work on Snow Leopard...
Apple's official list: http://support.apple.com/kb/HT3258
User-compiled more complete list: http://snowleopard.wikidot.com/
If you spend $160 (I don't know how many GBP that is), you would also be able to install/upgrade iLife and iWork in addition to Snow Leopard. This would be through their Mac Box set. iLife consists of iPhoto, iMovie, iWeb, and GarageBand. iWork consists of Pages, Numbers, and Keynote.
Hopefully this can be of some help. If you ask Apple, they would prefer you spend more money and buy their Mac Box Set.