I have no idea why you are having bandwidth problems. However I do not think that it is the 2820.

If you are routing all of your traffic via the vpn, your performance will be limited by your connection and the speed of the other end (In and Out)

You could always add routes for BBC Iplayer to go down the vpn, but all other traffic goes out via your wan connection. I do not know the routes that you would need.