If you can get Virgin, there's no doubt. They've just insalled a new router which is ready for 400mb - fast enough for proper HD TV.
If you visit the Virgin site and put in your postcode, they can tell you if fibre optical cable's available.
All the other providers use copper cables, which are Victoriann technology, and make silly speed claims.
Virgin's service is brilliant, too. It's free, (I think that's unlike Sky) and they don't repair equuipment, just bung in brand new stuff.
I don't work for Virgin, honest!