Why has it taken so long to be deployed? OFCOM and equipment availability.
Other probable reasons are cost and demand.
Back in 2001, ADSL was in the very early stages of roll-out (our exchange, serving 50,000+ people, had only recently been enabled for ADSL1 when this report was written).
Nobody really knew how many people would go for ADSL1 (then priced at around £40/month for 512kbps).
Most people were still on dial-up, or didn't even have an internet connection.
With over 80,000 cabinets (nationwide), the cost is tremendous.
If you assume, maybe £10k per VDSL cabinet (probably not too far off, when you consider the cost of the enclosure, the equipment inside, power hook-up, installation and commissioning), you're talking around £800million for the cabinets alone (could even be more).
Then you've go to add on the cost of installing maybe a quarter of a million km of fibre to serve these cabinets.
Then you've got to add on exchange equipment and back-bone infrastructure.
It's not really a surprise it's taken this long for a partial roll-out.
Also BT weren't too far behind other countries (most started rolling it out around 2008-2010). The big surprise was Russia, which started a roll-out in 2005!
vDSL2 FTTC Infinity with BT
DL Sync 40Mbps
UL Sync 10Mbps