What speed are you currently getting? Have you done a speed test?
Just to clarify as well, you are not getting 20 megaBITS per second, rather than 20 megabytes?
Also to note, was it up to 20 mbps, rather than a guaranteed 20 mbps. I believe there may be a minimum guaranteed speed, but they won't guarantee the maximum 100% of the time.
And unfamiliar with the exact wording of the contract, but is your guaranteed wifi speed different to a wired connection. While the majority of users will be using their wifi router for wifi, a wired connection is more likely to be reaching the max speeds advertised.
There will be reasons why you cannot get the speed - if they have assumed perhaps you have fibre to the cabinet, and your road's cabinet does not, that may explain it.
Your devices to a lesser extent may have an impact - you could have fantastic broadband that will pants if the wifi card in your device just isn't up there. Unlikely to factor at 20mbps though as far as know.
I'm fairly sure you can cancel the contract if you are not getting the guaranteed minimum, but you will still need to return the hub. Why can't you return it - if you've broken it - of course they can charge you - its technically their property. If its that you've thrown out the packaging, just explain, and say you will pack it in what you have. If its that you can't be arsed, well think of the £75 charge as a convenience fee for not having to bother with packing it up.