I’m guessing that the electrician is linked in with the Solar Panel deal. If yes, then I can understand their reluctance to put in a bigger one, as their time allocated for the job will be limited. They will also then have to test all your circuits on that CU.
I think in your position, I would accept what they are doing, as the cost of a CU is fairly cheap, and it gets the job done, then consider the tidying up of CU’s as a separate job with another electrician of your choice.
I paid around £650 for an electrician to swap out my old CU for a bigger better one, and to merge in the separate Conservatory CU. Then as a separate job he added a HiFi CU, along with 40 meters of 10mm SWA again costing around £650. The job ended up taking longer than he thought, so you may have to pay more, but I was very pleased with the job. I especially liked changing my old 30 year plastic CU with a single RCD, for lots of shiny MCBO’s in a. nice metal CU with SP.