If I may interject my hypothesis (though I've not been in the UK)...
Could it be that because the cost of operating a car in the UK, regardless of whether it's a 15-year-old beater or a new Bentley, is somewhat higher than in the USA and that it's often something more of a luxury to begin with means that the relative savings between a cheaper car and a more expensive car aren't as prominent.
This is the main reason why you see more luxury cars in Manhattan than elsewhere, even when you control for affluence: once you're talking about hundreds of dollars a month to buy a parking space, the difference between a car that costs $200 a month to lease vs. one that costs $700 a month to lease becomes rather less significant.
Similarly, high fuel prices, in and of themselves, are unlikely to dent sales of the long-wheelbase full-size luxury cars (BMW's 760Li, for instance) too much, even though those cars get in the low-mid teens for gas mileage. Many/most of the buyers of those are buying those for a chauffeur to drive them around in (the long wheelbase being primarily to get more legroom in the back), and the cost of paying a chauffeur reduces the relative increase in total operating cost (if you're using 100 gallons a month (60 miles roundtrip divided by 12 mpg times 20 days/month) and fuel triples from $2 to $6 a gallon and you're paying the chauffeur $2,500 a month, then the total goes from $2,700 to $3,100, a not-even 15% increase).