Hi
Depending on your installation method you could get away with a 63A MCB in the existing board if it has space, assuming your Zs at the new board is fine. You might get discrimination issues, but it’s hardly a critical installation that will cause life threatening issue if it does happen - you will have to have a think about that and any issues it may cause. Someone else mentioned fitting a fuse carrier rather than an MCB, thats a good idea if you can.
You could put a Henley block in and split the tails like you suggested, but there should be 1 point of isolation for the whole installation, so you might create more work doing it that way. In my experience, the majority of domestic customers want a cheap job rather than a well specced job.
Your 16mm T+E should be fine with regards to earthing, presuming its not being used as a bonding conductor. If it is being used as a bonding conductor then it will need to be sized appropriately. Is there services in the new area that need bonding, or are they supplied by plastic pipework?