I would suggest using C6A for lighting circuits. It may reduce
the chance of a filament lamp tripping the MCB. BS1361 cartridge
fuse is another option (probably a better option).

I'm not worried. I do not possess a filament light bulb in the house, except
in my Maglite. I prefer MCBs to fuses whenever possible, as it is easier to
reset at 2am, especially when you've got no spares.

I would also split the rest of the house ring main into two.

Strangely, the 1970 rewire did install two ring circuits, but they
were joined into one big ring in the CU, which also makes me think
the CU was older and not changed at that time.

Mine is similar, but not quite. The ring went down from the old CU, round
the ground floor and then up, past the CU to the upstairs and back down
again. I can split it simply by cutting the cable and crimping an extension.
I'm just too tight to buy another RCBO until I have to!

Christian.