All providers get their estimates from the same BT Wholesale database.
You'll never really know what you'll get until you've had ADSL2+ BB installed. Your actual speed will almost exclusively on the length of your line and will be much the same whatever ISP.
Note that estimate says nowt about whether you can or will be able to get FTTC. That may be cuz the local cab has not been enabled even tho' the exchange has. Wait till you get a phone # and then try again at http://www.dslchecker.bt.com/adsl/adslchecker.welcome
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 19 Meg WBC