Western Power would allow any length tails - They where happy as long as they where installed to BS7671.
However, I am not sure on the standing at the moment, as EDF has taken over as meter operator in the Western Power areas, so Western Power are only interested in the service and network, not the metering.
I am not sure if EDF allow any length of tail - I should really ask.
When moving a service, it depends on your SUPPLIER (B Gas, Eon etc) as to if Western Power can move it without contacting them or invloving them at all. In most cases they can, so I guess any length of tail would then be fine.
If however, the supplier wants to do the logistics then they will have to use Wester Power to move the service and EDF for the metering.......usually then involving a good down time to your supply, and you will be held to EDF's rules.
EDF's lads are old WPD lads, and if you know them life is easy. If they do not know you or your company you have to leave a BS7671 cert on site to show dead tests done, and they make you jump through hoops!