It has been announced with every speed upgrade that customers with Pace STBs need a hardware upgrade to achieve any higher. When the upgrades to 1, 2 and 10 meg took place, people who were on the £24.99 package received unlimited 1meg, as opposed to the alternative (cheaper) capped 1meg service.
I am not 100% sure what happens with the latest round of upgrades, but wrt having a cable modem fitted- an engineer will install it, and you'll need a PIN number.
Then connect everything up and visit
https://autoreg.autoregister.net/ and follow the instructions- or alternatively you can use the CD that will be provided if you prefer.