Your speed will not increase just because you have gone onto a different contract. What was the estimated speed range that the sales rep said you were on and what speed is it meant to rise to?
Can you run this checker and post back a screen shot of the results. Make sure you delete your phone number before posting.
and use this speed tester then scroll down the page and carry out the further diagnostics and post back a screen shot of all the results including your IP profile for up and down. This test must be done with a wired connection.
If you have a Homehub 5 can you post the stats from 1-12 by logging onto the Homehub management pages then troubleshooting > helpdesk. http://bthomehub.home/
You should, according to that estimate, get somewhere between 35Mbps and 49Mbps.
You should be getting that just now if everything is working correctly. It will not change just because you have re-contracted.
Can you post the other information I requested.