Couldn't care less is still used in the UK. Just another example of the strange transatlatic differences in the English language- e.g.

herb pronounced as 'erb in the US
US say Math and Sports whereas the UK say Maths and Sport.

Something that is declining with each generation on both sides of the Atlantic is the use of the th sound in favour of f e.g. Fursday and the increasing use (in certain areas) of Aks instead of Ask.