You need to consider the flag and the import duties / VAT as separate issues.
Let's take registration first as that is easy(er). One thing here is certain you are a British Citizen (according to your post) and therefore you cannot document the boat in the US. You can document it in the UK or any other convenient place, this makes no bearing on the VAT / Duty status.
Your best choice would be to document the boat on the UK PART III (Small Ships) register (note NOT PART I), this is quick, cheap
and very easy. It can all be done online and the documents arrive in a week or so with no need to provide any suplementary proof. Costs £25 and lasts 5 years.
Only snag for you is that you are meant to be 'resident' in the UK, so you may need an intermediary to handle the postage for you.
You can do all this here : UK Ship Register - Pleasure Craft/Small Ships
Note that you will need to 'flag' your boat before you leave the US.
Now on to the VAT / Duty issue. You *MAY* be able to take advantage of one of the very few loopholes left with Her Majesty's Revenue & Customs
If and only if, HMRC takes the view that you are moving your place of residence to the UK *AND* you have been out of the country for > 12 months *AND* you can prove you have owned the vessel for > 6 months, you should be able to import the boat free of duty and VAT as part of your 'personal belongings'. There are strict tests which HMRC apply to whether or not your are 'moving your country of residence', but unfortunatley, they don't spell these out in public. You could only get real guidance by contacting them directly and giving them your plans. Even then they reserve the right to take a different view when you actually pitch
up with your boat.
Here's the relevant wording :
If you are moving your normal home from a non-EU country to an EU country, including the UK, you may import your vessel free of customs
duty and VAT providing you:
- have lived outside the EU for a continuous period of at least 12 months; and
- have possessed and used the vessel outside the EU for at least 6 months prior to importation; and
- did not get the vessel under a duty/tax free scheme (see below); and
- declare the vessel to our officer; and
- will keep the vessel in the EU for private use; and
- do not sell, lend, hire out or otherwise dispose of the vessel in the EU within 12 months of importation unless you notify us first and pay duty and VAT on disposal.
Follow this : HM Revenue & Customs: Tax on personal items when moving to the UK
link for more information
Now on to the final hurdle. edsailing mentioned the RCD. If the boat you are looking at does not have a CE mark showing conpliance with the Recreational Craft Directive(RCD) and the documentation
to prove it, move on and look for another. Importing a boat into the EU without this can become *VERY* costly indeed. HMRC will want to see proof of this and without it, you will have to go through very costly 'type approval' on the vessel before they will even allow you to import it.
Hope this is some help