The £8.00 fee is actually the shipping charge for Customs to post it on to you after they have pawed it to death in every manner they can.
It's not HMRC who are levying that charge, it's Royal Mail:
http://www.royalmail.com/delivery/mail-advice/customs-information
It's a "Handling fee" and is payable on any package that has customs duty to be paid. Frankly, I think it's utterly obscene, but what can you do?
This mention of courier services sounds like it might be a better solution. Get your seller to ship to you using a courier who don't surcharge you for "handling"