Does this setup contravene any regs or is it ok?

[brianmoooore]

Just the first paragraph.
Reg. in question is (as someone else mentioned) to cover the likes of borrowed neutrals.
I have always designed domestic lighting installs so that if one light is out, then the next is not only on a different MCB, but on a different RCD as well. For instance, on a first floor, all the rooms would be on one or more MCBs, but on the same RCD, and the passage/stairs light would be on another RCD.
On the "more than one circuit into one MCB" argument, I consider it good design to bring lighting circuits near the consumer unit back separately and into the same MCB because of the help it gives to fault finding.
The very last fault finding job I did was to sort a s/c on a lighting circuit, and there were three "circuits" originating from the same MCB, affecting considerable part of the house. After a couple of minutes testing at the consumer unit, I was able to reinstate all but couple of utility room lights, which were left to the following week.

 

[Jjc]

I don't believe it's a regulation, but it is suggested that you leave a percentage of spares. Last one I did, it got 2 spares for 8 circuits. I can't remember the percentage though.

As for the question from @Jjc about identifying which cable is which... as @anthonybragg said... label it Simples And if that hasn't been done, used some basic fault finding techniques to identify it... link L-E on one, N-E on the other and a quick continuity check will establish which is which.... then you label it.

Yes I know all about circuitry and tracing circuits what about three phase boards if for some reason someone connected one of the two cables into a breaker
on a different phase? We are not talking simple domestics now.

 

[SparkyChick]

Yes I know all about circuitry and tracing circuits what about three phase boards if for some reason someone connected one of the two cables into a breaker
on a different phase? We are not talking simple domestics now.

Then it's two separate single phase circuits supplied by two separate OCPD in a three phase board.

 

[Octopus]

Is there such a regulation?

That's a very good point - wonder how many RCD's the OP has got in his double stacked board - more than 2?

 

[driverman]

Bit random: When designing an installation, should the distribution board/consumer unit have 25% spare capacity when installation completed?

 

[SparkyChick]

Bit random: When designing an installation, should the distribution board/consumer unit have 25% spare capacity when installation completed?

I can't remember the percentage, but there is a recommendation somewhere that you leave some spare capacity.

 

[Wilko]

No regulation that I'm aware of.

 

[Ian1981]

What’s the point of it if there was one?
If you fitted an 10 way DB for 8 circuits and someone came along and used the 2 spare ways then does that mean the board needs changing to 12 way?
It would never end.

 

[davesparks]

I think the regulations recommend that consideration be given to foreseeable future expansion to the installation somewhere near the beginning of the book.
I may be recalling this from previous editions though.

 

[Michael J]

BYB Definitions

Circuit. An assembly of electrical equipment supplied from the same origin and protected against over current by the same protective device(s)

314.4 Where an installation comprises of more than 1 final circuit, each final circuit shall be connected to a separate way in the distribution board ....

Neither of these state that a final circuit only has 1 cable

so............ I would say that the OP's original circuit complied 100%

Or the OP could have whipped out a short length of T&E, plus some Wago's and joined the 2 cables just before the MCB and then what