How old is the existing wiring? One of the reasons a new CU is surprisingly expensive, it that all the circuits connecting into it have to be tested, rectified if necessary, and certificated - that means a test and possibly overhaul of your entire electrical system; so if you're going to all that expense, it may be a good time to be rewired. If nothing else, it's a good time to split the sockets into an upstairs and a downstairs ring; same with the lights, and add any oddments.
I was thinking, the other day, if one just wanted a new CU and the existing wiring was adequate for now, one might ask the electrician to include a couple of 5A lighting circuits with only one lamp in them, next to the CU, and maybe a 32A ring and a 40 radial, both of which fed only one socket each, on the grounds that a householder can add more sockets, or lights, to a circuit without needing to be recertificated.