I know what you mean. My thinking is that there's a bigger market for a Lexus than an MG though. The thing with the MG is it's a high performance car that's cheap to run, so they've always attracted people on a budget. And people on a budget aren't going to want 400+ a year VED. With a Lexus there might be people with a slightly higher budget for running costs.
..if you really, really 'wanted' a Lexus would you be put off by an extra £200 a year.?., obviously the price will drop as a result of the tax rise but hopefully most people will counter the tax rise against what they will make from the drop in price..lets be honest the IS200 is hardly a practical motor, people buy them because they want them and if you want something bad enough 200 notes isnt going to stop you getting it is it!
But I can see the point on the MG, people buy them because they dont cost the earth to run. Guess people would be looking at other cars first in the lower tax band...