This is really useful, thank you and I will check with building control to see if that is a viable option.There's a number of points here.
Regarding the consumer unit - As far as I can tell that's a Lewden unit, parts are available for it, and there is capacity.
I can't immediately see any reason it would need replacing. If a lighting circuit has been extended then the lighting should now have RCD protection (see below). There's also a locking pin missing but that is common and fixable.
Regarding Building Control - is anyone actually asking for an electrical installation certificate?
I'm doubting that there is any notifiable work here as it looks as though circuits have just been altered and the work isn't in a special location. If that is the case there is no requirement to notify building control unless they are aware of the work for other reasons and are asking for a certificate. So you may not actually need one. It would be the electrician notifying building control and not you in any case.
Regarding the electrician - he is supposed to issue a certificate for the work and you are entitled to one.
If the electrician is claiming the board needs changing because the new lighting isn't RCD protected, an RCBO unit can be fitted on the right hand side, and these are available new and 2nd hand.
If the electrician is claiming that the consumer unit doesn't meet the latest regs with regard to division of circuits to avoid nuisance tripping then the lighting is separate to the sockets and that is acceptable.
He also shouldn't have proceeded if there was an issue with the circuit he was altering.
Finally, the building contractor supplied the electrician. I'd imagine the building contractor has now been paid?
In regard to your last point, yes unfortunately all payment has been made..