Presumably you are planning to buy a boat in the USA because of the extremely attractive exchange rate? It's worth adding up all the additional costs that wil be involved (travel to inspect prospective purchase, almost certainly repeat travel to complete sale etc, travel to collect and delivery across Atlantic, re-registration, alterations to fit the RCD and so on) and asking if it is such a bargain after all.
There are always bargain boats to be bought at Gibraltar, where some people's dream voyage-of-a-lifetime ends after a nightmare in Biscay. You might get a better deal by looking there: far fewer and lower air fares involved, and boat will probably already be CE compliant! Also you can start your adventures in the Med, which is probably a better learning ground for a new skipper than the Atlantic or the Pacific.
As to the VAT valuation, I think the Customs probably have a reasonable idea of what boats are worth. The reason they don't automatically work on how much has been paid is to stop people presenting a joke invoice: £30,000 for a £300,000 boat, say, and expecting to get away with it. If the invoice looks about right for the boat, they will probably accept it at face value.
Especially if it is to be your first boat, you should spend as much time between now and then doing as much sailing as you can on as many different boats as you can, to start to build up a picture of what works for you, and what you should be looking for when you come to make the choice.
Good luck with the hunt!