I've read in several places that the CLS 55 AMG model was only sold in the USA. However, I've seen many of the 55 AMG models in the UK and also for sale in the Auto Trader. How is this possible with the 55 AMG model only avaliable in the USA?

As far as I know, you definatly CAN buy a Merc CLS 55AMG in the UK.
I don't know where you read that but the only reason to ban a car from a country it's because it doesn't have enough safety.
For example. TVR is forbidden in America because:
--it has no airbag;
--it has no ABS;
--it has no Traction Control;
--it has no ESP.
And this, in car that has 350hp to 800hp is very unsafe, so the USA doesn't allow it.

But Mercs are safe, confortable, powerfull...Where did you read that again?