Personally I would fit a 10A or 6A MCB with a 1.5mm2 t&e, cable installation methods permitting.
Then fit a 3A BS1362 fuse in your DP switched fused spur.
Do you really mean an MCB? So that is a new circuit just for the boiler.
You will need a switched FCU in any case as most boiler MI's say you must have a double switch to isolate the boiler.
Generally you'd just installa switched fused spur, at 3amp.
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