Well I have mostly fuses in my customer unit which I wanted to replace with MCB anyway so might as well do it when I do this job since the power will be off anyway.
You do not need to change that!. The 40A MCB is there to limit the current on the 10mm cable. Nothing changes. The current to your sockets will be limited by the (13A) fuse in the FCU. The circuit from the load side of the FCU to the sockets will be a radial. a ring is pointless. EDIT Well you could, but your better strategy would be to change the Hager board for a compliant one (all metal case and all circuits RCD protected). You gain nothing if you swop the fuse carriers for MCBs. PS Let me know if you do get rid of the fuse carriers. I have a home for those!
Good point. So if I understand correctly, MCB is to protect the cables and you choose it based on what cable you terminate into it, not necessarily what you connect to the circuit (obviously you will always connect less than cable/MCB is rated for). Correct?
If it is a 10mm twin and earth cable I would put a 60 amp junction box onto the end of it, because they have 25 mm terminals and will easily accommodate the 10 mm, then run 2.5 mm T&E out of it to supply the sockets on a radial, then swap the MCB out for a B20. That way you can use readily available materials. https://www.cef.co.uk/catalogue/products/184450-60a-junction-box-3-terminal-brown
If it is a 6 mm T&E I would use the large Wago connectors in a Wago box to reduce to 2.5 mm, but you would have to buy a pack of connectors, so a smaller junction box would be a more viable option for you. Again with a B20 MCB. https://www.cef.co.uk/catalogue/products/111379-30a-junction-box-selective-entry-3-terminal-brown
You're probably committed by now but if not... consider a heat pump if possible at your location. RHI will likely pay for most of it.
Not sure why you are removing the electric shower as it is always nice to have a backup in case of boiler problems, but I am sure you have already thought about that. As others have said you could down rate the MCB and fit sockets. You could consider a 'lollipop' circuit if you want to have a 32A MCB.