How fast is the actual connection? It's best to start with a test using a cabled connection so you have a baseline.
BT should have told you how fast it was expected to be when you signed up. Those figures suggest that it's a 40/10 connection rather than 80/20, but it could just be the distance from the cabinet.
Whatever you do with the wireless it's never going to be faster than what you get with a cable. Your flashy new router should help with LAN transfers between your devices, but for Internet access it's massive overkill and as you've seen will make little difference.