Virgin Mobile are a virtual network operator and don't have a 3G or HSDPA network of their own.
They use T-Mobile to carry calls and data services and they have a joint company with Hutchison 3 UK called Mobile Broadband Limited which provides network services across the UK. 3 and T-Mobile share sites but still retain completely separate bands of radio frequencies for their customers. (Orange has recently merged in the UK with T-Mobile - 'Everything Everywhere' and will also be joining Mobile Broadband Ltd).
So the answer is that Virgin Mobile's HSDPA is dependent on T-Mobile's coverage which can be viewed on the T-Mobile site and this now includes a roaming agreement with the Orange network.
On a similar note many wil not be aware but Telefonica/02 and Vodafone have a similar agreement to share transmission sites in all of the European countries where they operate including the UK so this means that in the not too distant future there will only be two mobile networks to compare but there will be five network operators selling services and many virtual network operators (like Virgin, Asda, Tesco etc) also reselling the same mobile services.
