If by VoIP you mean literally 'voice over IP', then the short answer is
that the telcos will migrate their networks from being TDM-based to
voice over packet, i.e. the 'VoIP providers' in the UK in the future
will be the TDM providers we have now. The reason is quite prosaic -
TDM switches are getting more costly to make as they are based on old
technology. Softswitches are expensive to develop, but much cheaper to
make and use current technology.

With regard to current UK VoIP service providers, my impression is that
we are in about the same position as the US market was in 2000. At
that time, companies such as ITXC offered low-cost international calls
over the Internet. The customer base was dominated by expatriots
calling their relatives back home. The current UK market seems to
comprise those who are interested in the technology and/or those who
want cheaper phone calls, i.e. innovators and early adopters. I gather
that Vonage is now up to about 1M subscribers in the US, so this gives
an idea of how long it's taken for the VoIP market to grow there.

The challenge for the telcoms industry is to identify applications
which are going to drive deployment of a true broadband network. Given
that 7M homes in the UK have a satellite dish, do we want/need TV over
the phone line (whether it's copper or fibre)? If we have multi-MB
access to our homes, will we all buy videophones? We didn't when we
had ISDN2. Or are we going to start swapping our DVDs on-line? To sum
up, what's the golden application for mass-market broadband? If it's
TV >> Sport >> Football then why change the telephone network to do
something which Sky does already?

Nick