I would add that you don't have to have an acceptable EICR (no C1/C2) to sell, but the buyer is likely to haggle for a price reduction if not. Whether you give them a couple of £k off, or not, or chose to upgrade first, is down to you.I trained as a electrician in the 70s but have long since retired. Just had an EICR test done on my property in advance of selling.
My own suspicion is if they expect a modern house in terms of number/placement of sockets, type of lights, kitchen layout, etc, then they would want to rewire anyway, so the lack of CPC issue would go away when they do.
But those are decisions for you and the prospective buyer to consider.