Good point, Certa.
If the minivan has a 20-gallon (US gallons, not imperial) tank, that's 75.7 litres. If petrol's averaging out at about £1.30 a litre, that's £98.41 to fill it up, or $154.34.
The darn van only gets about 20 miles to the gallon, doesn't it? So if I'm not wrong, that's 400 miles per tank, which evens out for about 24p per mile, or $.38 per mile.
In comparison, right now in Yuma, AZ gas is about $3.28/gallon. That's about $65 a tank, or £41.
I THINK I've got my figures right, but please correct me if I'm wrong. I'd hate to be giving the wrong impression. Additionally, who knows - you might be able to get American-priced gas on the base.
One other thing, and I'm sure plenty of people are going to disagree with me about this, but if you are interested in assimilating, I would not move your car over. It'll make you stick out like a sore thumb, or at least it would up here in Scotland. However, this might not be a big deal to you, especially if you're only planning on being in the UK for a couple of years.
And for the price that its going to cost to ship the car over, modify it, and ship it back, the US gov't could probably buy you a car over here for the same amount. It drives me mad that taxpayer money goes to pay for something like this.
Sorry, I'm at home on maternity leave and this chance to flex my brain-muscles has left me positively high.