Do such mods nowadays have to, by law, be done by a qualified
electrician and/or done with buildings control approval?

It is "or". You can, therefore, do the work.

the under-floor connections had all been done with 'chocolate blocks'
for example.

You will need to replace the chocolate blocks with crimps or accessible
insulated junction boxes if you want the building control approval granted.
There may be other things that need to be brought up to spec if there are
bodges like this.

I assume you have planning permission to split into flats? There are also
lots of other building regulations issues, such as noise and fire safety,
which can be quite onerous to achieve.

Christian.