A great insight into VM's network by Walter, and just to add to it, the reason behind the vast differences is, VM's network has only been in place approx 20yrs, whereby they could take advantage of changes in technology and provide a fibre/co-ax hybrid network.
Also, the original owners (NYNEX), only cabled to densley populated areas, where they would see a quick return on their money. Something our very own Walter frowns upon when BT apply this same logic to VDSL roll-out.
BT's network on the other hand, has been in place since Noah built the Ark. They also provide a telephone service to
wherever you may live in the UK.