An interesting question 2SP.
The proposed "change of use" would be from Residential to Residential, so even if this is classed as a change of use, it's within the same classification. There should be no need for Planning Consent there.
Council Tax re-assessment... I would say yes, that would be needed.
Building Regulations might apply though. Not so much for the building work, unless some structural work is needed, but for the "separation between dwellings" aspect.
Electrical work could be carried out by a Part P certified electrician (no Building Regs there.)
It will depend on how insistent the Inspector is over the latest regs. (Soundproofing. Fire etc.)
I'm 99% sure that Building Regs would have to come into it.
Perhaps Buildersmate might pick this up. He may well know more on the Planning aspect.
If not, then in this case, I think you might have to ring your local Planning Dept. for their opinion. I wouldn't like to stick my neck out on the Planning issue.