It's all an artificial limitation, as is the 140mph limit on electric - that's the operating speed, not the maximum attainable speed.
There is a speed the Class 800 set will be able to reach which will be as fast as they can physically move on diesel and on electric power, as fast as the various factors allow - power, mass, drag, friction, rotational limit of wheel sets and traction motors, cooling limits for engines and traction motors. One (or perhaps more than one) of those factors will limit the absolute maximum speed the train can attain.
That's how a Class 91 locomotive managed 161mph and an HST set managed 148mph. The limiting factor on diesel will almost certainly be a lack of power rather than any other issue, on electric, it'll either be a lack of power, or it'll be the point at which centrifugal forces start causing motors and wheelsets to destabilise.