"Internet connections are like a chain: they're only as good as the weakest link, and many times the weak link isn't the ISP"
In most cases, the weakest link is the connection to the property. In the UK the majority of broadband users only have the option of using the copper wire to their house, so the speed will be entirely dependant on whether FTTC has been rolled out to their area, otherwise they are on basic ADSL where distance to the exchange is the main factor.
The BT Wholesale speed checker can pretty much tell you exactly what's available if you put in the phone number corresponding to the broadband account in the "Further Diagnostics" page.
So maybe just getting BT Wholesale to supply a database of highest/average/lowest speeds by postcode will be good enough for most? Virgin and other cable suppliers could probably provide similar data for their connections.