Hi,
No, there are no charges for upgrading your contracted internet speed, with any provider. Basically your fibre optic connection is capable of outputting much faster speeds than you currently have. The only thing that is stopping you getting a faster connection is that you are paying for a slower one. How Virgin do this is to limit or "throttle" the speed at which you can send and recieve data down to the limit you are paying for.
However, I'm surprised you want a higher speed, as the standard Virgin broadband speed is now 10mb per second. This is much faster than the average home user needs and more than satisfactory for heavy gamers etc. If you're not on this speed (they normally upgrade users free on their existing contract as standard when they increase the base speed), I'd suggest you phone up and "prompt" them. ;o)