you can't! they provide the local loop and most exchanges, the vast majority of ADSL and fibre is rebadged BT service. and they offer some very good deals with the Sainsbury's vouchers. their service can be crap, but then so are all of them, so throw them in the mix.
wireless is specified to run at around 50Mbps. there maybe higher speeds if both your laptop and the wireless router support, but dont expect it. or frankly require it.
to measure the speed properly you should do so from a wired connection directly to the router, with a CAT6 or CAT5e spec...