Assuming this is Electrics UK:
Highest load nearest the switch, except that non-RCD circuits have to go together in a block next to it, then the RCD, then the RCD-protected circuits.
Cooker, shower, Ground sockets, 1st floor sockets, 2nd floor sockets, immersion heater, ground lights, first lights, second lights, (non-RCD fire alarm) (garage) (various odds and ends) spare 1, spare 2 for example.
People used to say that you should spread the high-load MCBs, using the low-load ones between them. The idea was that high-load MCBs can get hot, so instead of bunching together a number of hot MCBs, you space them out to avoid heat build-up. Seems sensible but I don't think its current thinking. Maybe modern MCBs don't get so hot, or can withstand heat better.
edited to say, but replacing the CU isn't an amateur sort of job, the electrician will know how, provided you've already taken the time to work out what you want and discussed your list with him.
Someone will be along in a minute to say there are lots of booklets you need to read and understand, but even after that, you shouldn't get in above your head.