No return? If you mean it is a radial circuit and not a ring then that is fine and would usually be expected. I hope your electrician will be testing all circuits before connecting the new CU as required. This will highlight problems with the install.There is one 30 amp feed to the kitchen for the oven with no return.
While the lighting circuit may well require a rewire if there is no earth continuity throughout, this does not mean your whole house requires rewiring. As stated above the electrician will test the existing circuits and this will allow their existing condition to be known.To be honest, I am thinking the rewiring route isn't such a bad solution, since it will identify any other bodges that have been made.
OOI, I believe it fairly rare for a spark to ask a customer to buy the CU for them to install. Most sparks shy away from this because with the best will in the world, the customer doesn't really know what MCB/RCD arrangement would be required and why. Also, you quite often find that the customer in a bid to save money has bought something horrifically inappropriate from ebay or somewhere! Nothing worse than turning up at a job and finding that you've got to install some tat.
Is your electrician registered here? They should be...