You can't use the full 20Mbit from elsewhere. Whatever technology you use you will be restricted to only being able to receive files where you are as fast as the 20Mbit service can send.
It works this way You on the move <-768kbit-> Your Cable Modem <-20Mbit-> Internet
You can see where the bottleneck here is, you can't get your modem to download at 20Mbit then magic the data to you wherever else you are, you will be restricted by the upload speed of your cable modem, 768kbit/s.
Chances are that wherever you are you'll have faster direct access to the Internet than 768kbit/s and even if you don't going via home won't help.
Businesses can do it because they tend to have the same upload speeds as download speeds on their VPNs, this isn't the case with home connections especially Virgin Media whose upload speeds are very poor compared with download speeds.
You should consider O2 or Be both of whom offer up to 2.5Mbit upload speed, 3 times the speed of VM 20Mbit and faster even than VM's 50Mbit.
For simple remote access consider
http://www.realvnc.com/products/download.html