Just check that the BCO will give a completion certificate and not say
(as some are) that it's fine for you to do the work, but they have no
one qualified to inspect and so won't give a completion certificate.

Good easy book is the electrical section in Jackson - Day (Collins)
which you used to be able to buy as a separate book. Not sure if it's
updated for new colours but just write the conversion on the relevant
pages to make sure you get it right. (I've got a degree in electrical
engineering and understand all the theory, but I can still never
remember what a 2-way lighting circuit looks like and the info on the
leaflet in the package is not user-friendly.) If you add the on-site
guide you should be able to do everything provided you don't go outside
the table in the on-site guide. Then all you need to do calculations
wise is to check cable ratings given cable derating for insulation and
running together. If you stay inside the table then you can be pretty
confident on voltage drop and disconnection times.

Fash