Haha that's random, no reason why you couldn't cover a car over £20 years old :S I have a heavy duty coverzone mini cover, and have it on the mini pretty much the whole time in all weathers. It's 100% waterproof even in heavy rain, and has a very soft inner lining, so doesn't scratch the paint at all. It doesn't sweat either, as it's breathable. It cost just over £100, but it's the best £100 I've ever spent! I've had the cheap ones, they're useless in anything but light mist!
What he said ^^^
Not got one for the minis yet, (next purchase), but we have one on our VW camper van over winter. I like them as they do protect from rain, frost, bird shat etc. Get a good quality breathable one and you won't get any sweat and it should have a soft lining so it won't harm the paint work. Some of the better ones have under body straps as well, to keep the cover nice and tight so it don't flap around.
I'll be getting a couple of these in the near future, worth every penny I reckon
COVER