Boards are cheaper in the good ol' USA but consider the following.
You can get a 2004 board for $400 add onto this a $40 shipping cost (est) and the total you have to pay is $440 convert this to GBP at 1.56 = £280 sounds great.
Now for the bad news.
You will then have to pay import tax & finally VAT on the total cost (inc shipping), the import tax will average at about 5% (this could be 0%-5% it depends on how customs class the item, some industrial companies get hit with 85% import duty) VAT is currently @ 17.5%. With these taxes your cost will then be £348. This is roughly £3 more than the latest boards on sale at www.princes-shop.co.uk
As a separate issue if you do have a warranty issue with your board you will find it near impossible to return to a UK dealer. Support you local board shop, with more purchasing power they will be able to negotiate better deals in future.
Your final option is not tell customs that you are importing a board and risk getting caught. Under this scenario you would then pay an additional fee to the delivery service (royal mail, etc) to have them levy the taxes.