Since it appears that you are on T-Mobile UK, things will be a little different than from us T-Mobile US users. From looking at the T-Mobile UK website, it does look like you have what you need as far as the data plan. The web 'n walk gives you the unlimited net, and the BlackBerry email finishes up where that leaves off. As long as you can surf the net now with what you have, and receive and send emails, then it would sound as if you have the right plans. Here is the only link I could find about BlackBerry email on the web page:
T-Mobile (UK) :: email solutions
If you're still not sure if you have the right plan, call T-Mobile and ask, or look for other users on this forum who are in the UK. Sorry I could not be of more help.