Hi,
The topic has come up quite a number of times in the transport section of this forum. In short, it's only worth bringing over an 'interesting American car'. As mentioned above, the classic muscle cars, as well as modern American vehicles which aren't officially sold in the UK or other countries besides the US. If you have a what I call 'bread and butter' car i.e one which is available here, then it's simply not worth bringing it over unless it has some absolutely personal sentimental value for you.
There's a busy and relatively thriving USA car market here, I for instance imported myself an Anniversary Edition Ford F150 Harley Davidson from Texas in 2004 and kept it around 8 years or so. At many car shows, you'll see some fantastic imported American cars, trucks, SUV's etc.
Driving left hand drive isn't so difficult once you get used to it, there will be things like car parks etc where the machines are for right hand drive cars which will be a quirk, and your driving skills need to be honed when driving in traffic due to us also driving on the left side of the road. Parts are usually readily available from specialists who stock USA car parts and if anything specific is needed, they can be here in the UK within a couple days etc.
Which car is it ? if you do need some parts or more info, let me know as my business is within the Automotive sector.
Cheers, DtM! West London & Slough UK!