ADSL or virgin cable?... either way they are both going to be advertised as "up to 20Mb.... not 20Mb, if it's ADSL then it's dependent on how far from the exchange/box you are, if it's cable you should really get the full 20Mb but it's not guaranteed.
that said....If you are using wireless try checking again connected to the router using an Ethernet cable.
I switched to BT Infinity a few months ago and my speed went from 2½Mb to 33-37Mb.If this fibre optic service is available in your area,maybe an idea for you to check it out.