I know it seems to now be poular to transfer BT lines to VOIP providers,
(with great effort I understand) can the same be done with numbers on other
VOIP providers?

I currently have an account on Voipfone and would like to move it elsewhere
(if I can find another provider which meets my needs)

The service I get from voipfone is really really good business class VOIP.
would recommend for for incoming business voip line any day of the week.
But I use their call divertion service which is great but use it a lott (to
mobiles) and the charges are high to the mobiles I use (Three 23.5p/min inc
VAT) and my Global Sim a Lichtenstien Mobile at 37/p/minute. when I compare
the rates I can get from lowratevoip (.8p & 15p inc) or some others, I can
seethere maybe scope in saving money elsewhere.

It is quite amazing to see the difference in pricing to some desitination.
Of course you do get what you pay for, which is why I went with voipfone to
begin with and payed for a geographic number. but the difference can be
huge, to call Lichtenstien mobile for example, cheapest was 15p/minute most
expensive was voip.co.uk at 71p/minute even vonage is cheaper