Hiya!
Aless - £6-7K in GBP is pretty much the $10K USD amount - taking into account the 'usual' exchange rates of about £1 = $1.5 or 1.6 USD. Thing is, a car in the UK for £5-6K is usually a super mini sized car or the very low end of a small hatchback. In the US, I think $10K USD will easily get you a small sedan sized car.
Second hand prices in the UK do indeed differ for the exact same make/model of a car in the US. Usually here, it'll be worth less. However, direct comparisons are usually with 'premium' brands as the UK doesn't really get the USA 'bread and butter' cars. So usualy, this comparison will be between things like BMW's, Mercs, VW's, Saabs, Subaru's, Volvo's etc.
The reason? - simple - The USA consumer generally has more disposable income than a UK consumer. This equates also into the overall cost's percieved by individuals. Petrol is around 3 to 4 times more expensive in the UK thanks to the government slapping on about 65-70% in taxes on top of the petrol prices - so economical small cars abound here. Now I only speak from the 'premium' brands perspective here, and as such, second hand pricing for something such as a BMW 3/5 series or Merc C/E Class will be substantially higher in the US. This is becuase sellers know only those people with the money to run such a car will want to buy (or will buy) hence asking prices stay higher for longer. There's no such 'exacting' formula to all of this as some cars are more revered than others and often buck certain trends that are usually seen as the norm.
Chadwyck - cars that appear super cheap usually can be attributed to a few factors - Dodgy, salvage, repaired, distress sale. There are other reasons why a car may be cheap - mine is an example, it's an Ex Police Car, so had what's considered by 'ordinary' driving consumer as having a starship mileage for the year, and is a basic specification. If you're willing to overlook these and a few other possible elements, then buying an Ex Cop car comes HIGHLY recommended by me!
else, it's a case of being at the right place at the right time for the distressed sale type cars coming up. A damged repaired car can be a good money saver as well provided the repairs have of course been carried out to a high standard. Salvage, well, these are really only for mechanic or hobbyists and of course stay away from anything dodgy.
Private sales are of course usually where you'll potentially get the best savings and a chance to haggle - currently, large saloons/sedans, premium marques larger engined cars are going for very cheap money due to the credit crunch etc. I suggest you delve around all the publications, figure yourself out a shortlist of maybe 3-5 cars and then go take a look at a few of them and take test drives etc. Make sure you check out the Cargiant supermarket in White city, the Trade Sales supermarket here in Slough! and the supermarket place in Chertsey (forgot name of company) at least here you can walk in, ask for a test drive and walk around looking all serious and then leave saying you'll comeback to enable you to get a feel of the cars on your shortlist. They are good places to buy from as well.
Good luck !
Cheers! DtM! West London & Slough UK!