If the mcb's are obsolete, it seems a waste to spend money on a 2 way board for your extension. Supposing an mcb fails in the near future on your current cu - you would then have to change the cu anyway. Why not just get a new cu with plenty of spare ways , and a good branded one, and then you'll never have any problems.
At the minute , tlc are doing 17 way Wylex units with RCD split for about £40 , and the mcbs are £3 each, they also do versions with 9 and 12 ways also, but get what suits you! If you dont want do fill all the mcb slots, they also have the blanking plates for them. And you have room for more expansion.
If you work out what it would have cost for a seperate 2 way cu, the henley blocks, 16mm tails & earth etc , I think you'd find it to be a much better (and neater) approach!
If you want to get it up and running for the time being I suppose you could have always add the new ring to your current one (and this could now easily be done at the consumer end by removing one of your current cu ring cables , joining it to the new ring circuit with an accessible junction box , and put the other end of your new circuit back onto your cu where you removed the other cable) ! Again this approach is house size dependent and depends how much loading you already have on your sockets.
You could also add your new lighting circuit to the consumer, providing you dont have too much load on your current lighting circuit, by just adding it to the 6a mcb thats already there!
I'm very surprised that building control has'nt forced you to get somebody part p in (or even came out to inspect the work!!) I know they do like the extensions to be on seperate circuits (and seperate consumers) , but seen as they seem to be happy letting you carry on with your own thing, and obviously as they are'nt inspecting it, then its down to your choice , although they will probably want some paprework and test certificates etc.
These are just general views, I'm not saying ' This is what you HAVE to do!!' Seriously though - Consider the new consumer option.