The cost of parts compared with cost of labour is not that much, but an MCB is probably £2.30, and a RCBO around £16, and an RCD around £21 so 2 x RCD plus 8 MCB = £60.40 and all RCBO around £128 so maybe £70 extra to have all RCBO, so that is a small part of the £800 total, it will need a surge protection device (SPD) and the tin box that they all fit in, glands and wires, so those figures are not whole cost of parts, but the difference between a good job and a get it to comply job.
There are also different makes, I looked at Fusebox, not at one of the traditional manufacturers which tend to charge more, the cost of the SPD can vary a lot. But the work involved is around the same, and the only reason for selecting a high-priced manufacturer is when something you need is not available from the cheaper manufacturers.
A consumer unit is a type tested distribution board, and that type testing means you can't mix and match, if you want a 24 volt bell transformer to fit in the CU, and Fusebox does not make one, then the whole cost can spiral just to get the one part.
I would hope today no one would fit a duel RCD board any more, but to compare prices it has to be like for like.
The electrician often goes in blind, he has no idea what he will find, it may be straight forward, but he could also have a load of corrections to do, in my house we had neutrals mixed up on one light switch. To test earth leakage is easy,
a quick check with my own house shows 8 mA leakage over 14 circuits, but this does not show borrowed neutral and the like.
Also, the faults which are not down to the installation, my freezer goed into defrost cycle around once every 30 hours, if there is a fault, it could take 30 hours to show, which clearly no electrician can sit there and wait, so it is taking rough with smooth, some jobs and done and dusted and he never returns, others he will have to return many times, for faults which are not really his fault, but still had to charge for. So he can do some testing, this meter
can help, but it would take a huge amount of time to do a full EICR (electrical installation condition report) and PAT test everything so the electrician has to take a chance, win some loose some.