Except if you actually read the linked page it says that it's at least partially due to the 7% fees have to deal with when converting the BTC back to sterling, which is all they're really interested in.
Add on whatever fees you pay to get hold of the BTC in the first place and you have quite an expensive system.
of course, there really is no need to pay fees to exchange BTC for GBP. Posting a sell ad on bitcointalk.org is likely to net you someone who will take them off you for between 2 and 5% above mtgox daily average.
Ah, so an unofficial/casual exchange method, with higher risk. Awesome.