current regs obviously require RCD protection of essentially all sockets, all lighting circuits and most buried cables, to name but a few things (and not leave much! ) - even in a TN installation, don't they?
Yes correct, but regulations are not law, if I pay my fee to LABC they can issue a completion certificate even if it does not comply with BS 7671, which is what you need to do if for example you wanted to install German sockets in the house, as our regulations don't permit them, but in 2020 we were still in the EU so the LABC had to permit them, even if BS 7671 did not.
It seems unlikely you would persuade a LABC inspector to allow you not to fit RCD's in some form, however the one time I used the LABC route, they never inspected the completed job, so they would have been unaware if I had not fitted RCD protection other than the installation certificate which would show no trip times for the RCD.
But
I will get the tennant to send me a photo of the consumer unit.
So there will be a copy of the EIC or EICR with the tenant if in England, which should show if RCD of some type is fitted.
it was always hoped that the lighting would be on a non RCD protected circuit so that you could still see to look for whatever had tripped the circuit.
Not any more, if there are lights in the bathroom they need RCD protection, even if wired in Ali-tube so the cable buried in wall did not need it.
It seems to me flawed to have a home so if any one gets a shock from some thing supplied from a socket outlet, they will also be plunged into darkness, and back in 2008 BS 7671 said:-
Every installation shall be divided into circuits, as necessary, to:
(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit
(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation
There was an argument at the time that a RCD did not form a circuit, and traditionally the 100 mA RCD used with TT supplies did have one RCD for all circuits, as did the ELCB-v before it. But the circuit = An assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s).
And the RCD is current operated, and if the current differential goes
over the stated limit, it trips, so my English puts it as an overcurrent device.
Of course the DNO fuse is also an over current device, but the MCB/RCBO after the fuse ensure the DNO fuse will not normally rupture due to a circuit over load. But there is in the main nothing after the RCD which will trip faster and with less current to the pair normally fitted in the consumer unit, yes I have a 10 mA socket, but the time is still 40 mS so both the 10 mA and 30 mA would trip for most faults.
So I fail to see how a two RCD consumer unit has complied with the regulations since 2008, however it has been pointed out many times that many people don't get nuisance tripping, even with 4 circuits on the same RCD, we should test the leakage and it should be less than 9 mA, but until a few months ago I did not have a meter able to measure that low, most clamp on meters seem to show 0.01 amp, but few show 0.001 amp, and the insulation resistance measurement is useless for this, as it measured with DC, and we use AC which can have capacitive and inductive linking, as shown many times when LED lamps still glow a little when switched off.
I know total leakage on my house is around the 20 mA mark, easy to put clamp meter around the tails, I have never opened the CU to see what each circuit measures. Since split between 14 RCBO's unlikely any one is over the 9 mA, (1/3 of RCBO rating) more worrying is the 12 mA DC, but it is so hard to zero the clamp on with DC measurement not really sure that is a true reading, but I have all type AC RCBO's although they said type B on the boxes, clearly referring to curve B. 
