cars built after a certain date (March 2001 i think) are already taxed
on different rates according to their emissions.
cars build before that are taxed by engine capacity - and the landy
already falls into the largest scale bracket (unless old enough to be
tax exempt).

As a percentage of cars on the road now, the number of tax exempt ones
can't be that significant, though i'm sure theyd still love to scrap
it and claw a few more quid from us.
I can't see it happening for a few years though since I doubt they
want to get any more unpopular with motorists.