if you sell a vehicle to a brit that is an american spec do you have to pay the custom tax? if so how do you go about doing that? any advice? 

if you sell a vehicle to a brit that is an american spec do you have to pay the custom tax?

Yes! The taxes need to be paid....it can be quite expensive. I can't answer the other questions, but I do know the taxes needs paid because a US Spec vehicle brought over via the USAF/US Gov for USAF personnel comes over tax free. If that vehicle is sold to a local then those taxes that were previously exempt need to be paid.

To find out how much the taxes will cost...take the VIN number to Pass & ID and they will find it out for you. 

This link might help:

http://www.mildenhall.af.mil/news/story.asp?id=123178479

Specifically this paragraph:
"Who has to pay the tax?
If you sell your car to someone who's not qualified to own a concession vehicle (or another concession vehicle), you are responsible for paying the taxes. You will need to contact the Pass and Registration office who will complete the necessary forms to have your vehicle assessed. Try to do this before you sell the vehicle, because once it is sold to a non-entitled person, the taxes must be paid. If you fail to pay the taxes at the time of the sale you'll get a bill later from the British government. There's a good chance it will come long after you've lost touch with the person who bought your car. If you want to negotiate the taxes and duties as part of the selling price, you can, but you must be prepared to pay importation taxes and duties on the car you've sold."

I would mention, however, I've heard something about being able to reduce the amount paid after a year or registering it on the economy and avoiding the cost, but I don't know much about that. 

Why would a brit want an american car? 

Why would a brit want an american car?

Probably a model not produced here?

Mustang, Dodge RAM, Dodge Dakota, Dodge Challenger/Charger etc....anything not offered here would be particularly sought after....especially if it was purpose built, like a "big" truck.

A "big" truck here in England is a Brit spec Ford Ranger or one of those odd looking Mitsubishi things. Back when we had our US spec Ford Ranger (XL, regular cab 2WD) we ALWAYS had Brits tell us it was a "Big truck!". Lol. So, maybe it is a situation where the OP has a US Spec vehicle that is sought after here in England.