If it's fibre all the way, then you're laughing and you can go with the BT line - however, if it's just fibre betwixt house and the box in the street, then copper to the exchange, then the extra bit of fibre will give you little benefit.
For the sky option, then you get crap upload speeds but great downloads speeds IIRC - has to use the phone line for upload but the dish for download. I never tried it in the end as the sky fitter man took one look at my roof and decided we couldn't have sky fitted as he was too scared to go on it.
Virgin's fast - fibre all the way, so no speed loss. However, virgin's customer service is second to none for being shit. Don't go with them unless you really like spending hours on the phone listening to alt-pop.
Personally, I've just got a bog-standard phone line, and an O2 broadband over ADSL on that. It's been fine. Don't know what speed I get. But don't download gigs of pr0n either so