First I really like your Raleigh 20 site. I'm sure the bike is even better to ride than to look at.
Second, I'm not sure if this is any help but whatever import duties are charged have nothing (directly) to do with what taxes or VAT were collected in the country of origin, but depend on how the UK charges duties on goods brought into the UK. For answers to that take a look at http://customs.hmrc.gov.uk
under "individuals and employees" cllick on "travel information" then click on "shopping on the internet" (yes, I know that's not how you're necessarily getting the wheels) and then click on "notice 143." I don't really have the patience to read through that notice but it seems like it at least starts to explain what you want to know. It appears that gifts and used items are charged duty and that you won't be able to take delivery until the duty is paid.
Good luck, and be thankful you're not in Italy (at least for customs purposes), where I am now and where customs delayed a shipment of my own books while I had to fax them numerous (three) declarations saying that the books were my own, were for my own use, would not be sold in Italy, and would return to the US before I did. In fact, to the extent that UK customs officials are more helpful than those here (they could hardly be less helpful) you might be able to get an answer by calling HM Revenue and Customs.
Please post pics when you get the wheels on the bike!