Just a thought whilst im still up and about, you may wish to check the BT Wholesale line checker to see what speed they estimate your line can handle.
http://www.dslchecker.bt.com/adsl/adslchecker.welcomeIt is the internal system that BT use to check and estimate your line speed when you order your ADSL service. It does tend to be quite conservative, so can be a bit lower than what you will actually get, and on occasion it can be a bit higher of course, but within reason, it is relatively accurate.
You might also wish to check with Be*, if you are considering o2 Broadband, their checker is fairly accurate as well.
https://www.bethere.co.uk/web/beportal/homepageIt tell's me the following:
You can get BE broadband on 0151678****
It's probably going to be this fast:
10 megs
Not bad, im seeing about 11 - 12Mbps sustained, so its not far off (im on the 8Mbps package, but o2 have neve restricted my sync speed for some reason).
Telefonica o2 have owned Be* for a few years now, and their o2 Broadband service uses the same network and infastructure etc, so it is essentially a line checker for o2 as well (Be* is the geeks internet service, o2 is their mainstream service).
As Tony says, o2 offer 3 months free, plus they offer a 30 day money-back guarantee, if you are not happy and cancel in the first 30 days, they will end your contract without any hassle, meaning you are not going to be tied into a service that is not for you.