Sites which restrict access to a particular geographical area do so by checking your IP (Internet Protocol) address - the address allocated to your PC by your ISP when you go online. If the IP address cannot be verified as, in your case, having been issued by a UK based ISP, you won't be able to access the service.
The problem usually arises when "Brits" abroad want to access services in the UK, but can also be a problem with users in the UK with ISPs such as AOL or Orange, which might allocate non-UK IP addresses.
I have Virgin Cable and on one particular site I visit, the location of my IP address cannot be identified. I haven't so far had a problem though accessing BBC iPlayer. If you are on Virgin's Cable service which, unlike ADSL, tends to retain the same IP address for a long time, you could contact Virgin and ask them how you can change your IP address to one that is clearly identifiable as UK-based.