Can you confirm your talking about the double pole red switch located to either far left or right of the consumer unit or the RCD.
This type of fault is normally associated with a RCD, which is identified by looking for the word RCBO or RCD on the module and double the size of the other breakers within the consumer unit.
From what you have suggested its a fault within the consumer unit, as the fault is not related to a specific appliance, nor a specific plug socket.
Assuming its the RCD this is most likely a faulty RCD which is incorrectly detecting earth leakage, hence its not exceeding the 32A rating for the circuit fuse to trip, but some voltage going to earth and tripping independently, a new RCD costs around £15.
Should the red double pole switch be tripping without any of the other circuits tripping this points towards the switch thinking there was more than 100A being pulled across all the circuits and tripping, a new double pole main switch is around £10.
If you would like an electrician to attend to replace the fault consumer unit module please provide me with your postcode.