hmm... well, i'm not sure how the broadcasts work in England, but in the US we're making a big transition to digital over-the-air, so the picture is literally perfect high-def as long as you have a decent signal. They do still pick up analog signals, though.
There is an alternative to these TV tuner cards called
slingbox... they work much differently, though. In short, they allow you to watch your home satellite or cable box (or any other video source, really) anywhere you have high-speed internet access. There is also more of an installation process with slingboxes than TV tuner cards. Anyway, just thought i'd mention it.