Hi Jammy,
Before you buy, just bear in mind that the up to date charts are really expensive (And chances are you'll still need to correct some, though they release pamphlets that give you the lat/long of new hazards. Chandlers usually show their corrections at the bottom)But anyway, the site I would use is the UK Hyrdographic office, they sell quite an abundance of charts, and of course, they're a government site.
http://www.ukho.gov.uk/ProductsandServices/Pages/Home.aspx
Also, if you're actually wanting to do some chart work...http://www.force4.co.uk/ - I bought some of my stuff off there.
Hope that helps!
Jack.
p.s. MCG, in that case, you'll be horrified to hear that there's currently a big on-going discussing about passing a law saying ships do not have to carry paper charts, instead, electronic charts. Apparently the Gov want them eradicated by the next 20 years!