A comparison site has, in an admittedly interesting case of stating the incredibly obvious, estimated that the average speed of customers taking BT Infinity’s new fibre optic broadband services, specifically those covered by FTTC’s initial 30 "phase one" rollout areas (original list), could be boosted by as much as 20 times.

Top 10 Broadband used speedtest data gathered from its StreetStats technology to localise the average download speed performance delivered with existing broadband technology. It then appears to have compared this against the advertised rate of Fibre-to-the-Cabinet (FTTC) technology, which promises download speeds of up to 40Mbps and uploads of up to 10Mbps.

Current average download speed (Mb / per second)
Basingstoke 3.6Mbps
Belfast Balmoral 4.7Mbps
Bury, Greater Manchester 3.1Mbps
Caerphilly 2.5Mbps
Calder Valley 2Mbps
Canonbury 6.8Mbps
Cardiff empire 5.5Mbps
Chelmsford 3.9Mbps
Chingford 4.6Mbps
Dean, Edinburgh 5.9Mbps
Didsbury 6.3Mbps
Edmonton 5.1Mbps
Enfield 5Mbps
Failsworth 4.6Mbps
Glasgow Halfway 3.4Mbps
Glasgow Western 4.2Mbps
Halifax 4.1Mbps
Heaton Moor 5.1Mbps
Hemel Hempstead 5Mbps
Leagrave 5.1Mbps
Luton 6Mbps
Oldham 4.3Mbps
Pudsey 4Mbps
Rusholme 5.1Mbps
Taffs Well 5.9Mbps
Thamesmead 3.6Mbps
Tottenham 6.6Mbps
Watford 4.6Mbps
Whitchurch, Cardiff 6.6Mbps
Woolwich 4Mbps

The average speed data is interesting but using FTTC's advertised maximum as a basis for estimation is very questionable. FTTC is a variable technology that uses VDSL2 over existing copper lines to deliver its broadband service. As a result BT claims that the minimum consumer speeds could be 15Mbps; any lower is likely to be considered a fault.