Consider the reasons for why you want faster broadband.
I used to use 3Mobile Broadband and found it far too slow - iPlayer would not work very well. Now have O2 and get 650kbps typically as max download speed (occasionally it peeks just above 700kbps). This is currently fast enough to watch BBC iPlayer.
Consider your traffic usage as well - unlimited download packages can cost quite a bit, whereas many ISPs cap the download to a max of x GB per month.
Compare ISP packages using a comparison website such as www.thinkbroadband.com www.broadbandgenie.co.uk
Also consider what is available from companies you currently use. For example O2 discount Broadband for their mobile customers. I found it cheaper getting a PAYG O2 mobile and topping it up every 3 months, than paying their non-mobile customer broadband rate.
I can't find any Mumsnet Discount Partners offering broadband deals (Note to MNHQ: Why are O2 not offering a deal, given the Joggler review they are doing?)
Also don't forget about Cashback websites like www.quidco.com - At time of writing this: VirginMedia are offering upto £120 cashback. O2 are offering £50 cashback for a broadband order fully done online (note: I did this recently and it was really easy).