Oooh, we are re-thinking at the moment, too! BT seems to get more and more expensive
To make our decisions, I'm planning a big session with uswitch, which I used when helping my MIL to get broadband and phone installed last year.
Which? can also be a good source of info about utility providers and broadband/internet packages. We started subscribing when they had a £1 offer for 1 month, which is really worthwhile if they still have this offer running.
IIRC, you have to draw up your comparison list to INCLUDE the cost of getting an engineer out to connect the line (and broadband). If the company pays this, do check whether there is an extra penalty, of repaying this amount, if you cancel your contract. uswitch is quite good about highlighting the full first-year cost, which includes setup and engineer fees.
I have used a mobile dongle (Vodafone PAYG), and it is good, except you can't stream tv, and downloading things from the BBC iplayer seems almost impossible, so you have to consider your internet usage to decide whether a dongle is the right thing for you. Or you may decide that you will be in an internet cafe frequently enough to take your laptop in there and do your big downloads using their wireless....