There are kinetic light switches, the moving of the switch produces enough power to send a radio signal to the master, so you can have two way switching without any cables between the switches, I think around £60.
Following the regulations it has never been permitted to borrow a neutral (in real terms it is the line which is borrowed) however in the 70's and 80's people started using down lights and the ceiling rose is used as a junction box, and rated 5 or 6 amp, so lighting circuits were limited to 5 amp (fuse) or 6 amp (MCB) so people started to split the lighting into two circuits, often they did not realise by doing this they had produced a borrowed neutral.
As we started to use RCD protection we started to find these borrowed neutrals, although putting both lights on the same RCD will stop them tripping, the regulations require them to be on the same circuit, so must also be on the same MCB.
It has come up many times about the problem that if a fault trips a RCD feeding a socket, you don't really want to also be plunged into darkness, however for many years houses with TT supplies had a single 100 mA RCD, and before that a single ELCB-v and caravans and boats still have a single RCD. But with AC there is always some capacitance and inductive linking, so there is always some leakage to earth, from previous posts it seems this is limited to 9 mA, however personally I have never measured the leakage, I don't have a clamp on meter that will measure 1 mA so all I have done is test with an insulation tester at 500 volt DC, so that capacitance and inductive linking has never been measured.
So as the designer, I simply put every circuit on its own RCD by fitting all RCBO's, (RCD and MCB combined) but what we want is if a socket circuit has a fault, we can continue to use the home, we don't need some one to fix it quick as the freezer is defrosting we can use an extension lead to keep things running, clearly don't really want extension leads running up or down the stairs, so home should be split front/back or side to side rather than upstairs down stairs, but as long as the cooker supply and kitchen supply are on a different RCD it is not really a problem, or of course kitchen ring and ring for rest of that floor on a different RCD.
However if the lights are split up/down, and the sockets split front back, then only way is really three RCD's. Be it lights on a RCBO or sockets on a RCBO to do all we want for the design some way we need three RCD's.
So in other words if you have not got a high integrity consumer unit, some how some way you need to compromise, and OK we are working on a budget, I may have spent £250 on my consumer unit plus fitting, but not everyone can afford to do that. So putting all lights on the same MCB will cure you problem, it is not ideal, but with only 2 RCD's there is no ideal fix anyway. If it is a high integrity board then put lights on a RCBO, if not the only question is do you put lights with upstairs or down stairs, personally I would put with up stairs.
I am not saying as wired your house is wrong, what I am saying it is a compromise, it has been done on a budget, so accept done on a budget and combine all lights on one circuit, or as said to start with use kinetic switches.
Yes it would be better if you do get a fault on lights not to loose them all, but if that is really a problem, then fit emergency lights. I have a rechargeable torch at top of stairs, so if there is a general power cut, it lights the stairs. It does not matter how many circuits you have, a general power cut and still no lights.
I agree this should be explained before the consumer unit is fitted, maybe it was, but all lights on one MCB with LED lighting is not really a problem, unlikely today to use over 6 amp on lights.
