Ideally you don't want the kitchen CU off your existing RCDs as the leakage on those ovens is likely to trip it taking out the kitchen and some of your home supply.Would this single cable come from the spare 32A breaker on my existing board as this wouldn't be enough? I like to understand what's happening and having done a little bit of looking I've seen that henley blocks are used to install sub-consumer units?
If your CU allows internally a 63A (or maybe 50A) MCB off the direct supply (i.e. from main switch to RCD input, not RCD outputs) then it would be better, but still not offering much selectivity. In other words if you have a big fault on one kitchen circuit it will also trip the supply MCB taking out all of the kitchen electrics. So don't put your lights on it with this configuration as that would be very dangerous. Probably you should look at some sealed "non corrosive" LED strip-lights with built in emergency backup anyway, just in case you are running lots of hot appliances and the mains fails plunging you in to darkness.
That is why the usual suggestion is to split the tails feeding your main CU (where the Henley blocks come in) and then have a fused-switch to feed the sub-main cable to the 2nd CU. The selectivity between a 63A fuse and something like a 20A MCB or RCBO for the cookers is quite good so a fault would be localised to the final circuit.
Generally you should have all of the final circuits on RCBO if you can, though if the ovens are hard-wired (e.g. FCU) and the cables not buried < 50mm they could have RCD protection omitted. Socket outlets should always have RCD protection.
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