BT covers almost 100% of the UK:
East100%
East Midlands100%
London100%
North East100%
North West100%
Northern Ireland100%
Scotland98%
South East98%
South West100%
Wales99%
West Midlands100%
Yorkshire & Humberside100%
And it isn't just 1 area, NTL or Telewest don't cover even 50% of the UK, since it is so close, if ntl:Telewest were to cover the same amount as BT, they would a lot more ahead then BT in customer numbers.