I disagree.
The OP has either disconnected or turned off everything on the affected circuit yet the MCB is still tripping. With everything turned off or disconnected, that live feed out of the consumer unit is literally connected to nothing. Nothing at all you could do to neutral or earth would cause any current to flow down the live conductor.
The fault has to be on the live conductor. Either somewhere in hidden infrastructure, or something has been missed during the 'isolating' session.
As said a neutral conductor is a live conductor, it carries current. In this instance there could well be a fault live to earth. But the fault could also be between neutral to earth, that could also trip both the RCD and mcb, as could a live to earth fault.
The fault could be in the fixed wiring, replicated by mechanical pressure, a pinched cable under floorboards for example. It could be a fault within an appliance, that has not be disconnected, and which is only replicated when the appliance cycles, for example a fridge or freezer.
I‘ve come across various faults in my time.
Examples; a strapper cable between two way lighting, one wire in the cable had been nailed into. The fault was only discovered, when the adjacent bathroom was being refurbished and the plumber was getting shocks off the wall when touch copper pipes. Fault finder was also complicated by the position of two way lighting switches.
Another was a pinched neutral cable on a rarely used landing socket. Only replicated, when a high load appliance was plugged into the socket.
Another, where a diy’er had installed a smoke alarm, and had connected the cpc into the interconnect. The RCD failed to function under test.
My point is, we can go round the houses, pontificating the problem. In this case the OP has tried the obvious. He now needs to leave it with a competent electrician, with suitable experience and test equipment, who should carry out test procedures as outlined in BS7671.