Virgin Media has been offering HD Services for quite some time, the catch is that it is not in all area's.
It is really dependent on the infrastructure available as to which services are offered and where - I used to live on the isle of dogs in docklands and could only get Analog cable not even digital (Let alone HD).
As you are probably aware the Virgin Media infrastructure is an amalgamation of different networks brought together - e.g. ComTel was bought by NTL, Telewest bought Cable and Wireless (and a few others). Telewest and NTL merged and then got branded by Virgin - so there are quite a few disparate networks under one umbrella.
The end result is that some networks are older than others and the equipment installed restricts the capabilities on each network. It all depends whether the subscriber base in your area means its financially viable for them to upgrade your local network to have the latest capabilities. If the uptake of HD and Hi bandwidth cable modems would be high (Premium products) then they may consider this - otherwise they would lose more money upgrading the physical infrastructure than what they would glean back off their subscribers.
With regard to HD Services, I have had SKY+ HD for a number of years - the majority of the HD channels are sky's premium channels (e.g. Movies and Sport) - I think at last count there were around 15 HD channels some of which (e.g. BBC HD) dont transmit all the time. ITV in HD is not available on SKY HD, Channel 4 is and a few documentry channels - the majority are sky's own channels.
It may therefore mean that unless your upgrading to the complete packages the extra £10 a month on a normal Sky Subscription for HD is not worth it.
More detailed discussions of TV's, HD and Sky can be found on
AVForums.com Home and
Digital Spy Forums - Entertainment and digital TV news discussion if you want to do some research on whether a switch of provider is suitable for you - you may also find more information on the capabilities of your local cable network and future upgrade plans there.
Cheers