I'd say, unless it's a classic/unknown car that you are desperate to keep, I wouldn't bother shipping it over.
Not only would you have the shipping costs, but also the cost of making it road-worthy and legal in the UK. Also, the insurance on an imported car may be really expensive (plus, if you have a US licence and no UK driving history, it'll be expensive anyway), and if you need parts/repairs, they may also be expensive/hard to get hold of.
Used cars in the UK are very cheap compared with in the US - my family have bought used cars for anywhere between £500 to £3000 over the last 15 years and they've been reliable and very good value for money.