I propose to do the following:

Re-wire upstairs power circuit
Re-wire downstair power circuit
(because they're both totally inadequate!)

Leave kitchen power circuit because it's perfectly adequate (the
kitchen is a 1970s extension)
Leave the lighting circuits alone (again, the wiring is fine, and the
circuits are adequate)
Leave the cooker circuit (no electric cooker, cable seems very recent)

Completely re-wire the garage


Replace the 6-fuse consumer unit with a split load 13 or 20 way
consumer unit. RCD everything except lighting and garage (which will
have it's own CU with RCD).

My question is: is it OK to connect the old wiring to the new consumer
unit?

I know I need to test it to check that it won't trip the RCD (is
20Mohm+ from the earth to phase and neutral, measured on a cheap
multimeter, good enough?), and I'm happy that the wiring is in good
condition, but I can't be sure that the installation of all this
wiring meets new regs. (e.g. The earth bonding in the bathroom is
non-existant, but I will fix that).

Does all the wiring connected to a new consumer unit have to meet
current regs, or is it OK to transfer older wiring (late 1970s,
possibly not quite meeting current regs in all respects) to the
consumer unit?

Cheers,
David.