I would say poor bleeding is most likely - it is possible that the thermostat is stuck, but they are more likely to fail in the open position, in which case you are going to get the car running below normal temperature rather than overheating.
The temperature sensor is also unlikely to cause such an issue if the coolant system is otherwise operating correctly - the sensor will only come into play when the car is stationary or moving very slowly as it triggers the radiator fan to start up. It is very rare for the fan to need to come on when the car is travelling at any reasonable speed as the air flow through the grille is always sufficient.
In any case, if the head gasket has recently been replaced by a professional mechanic, it will be warrantied, and your first course of action should be to take it back to him for rectification. It may be something simple such as the correct bleeding procedure not used (the K series can be difficult and Rover always recommended using a vacuum fill system), or it may be that there is something more serious awry - even 'professional MG Rover specialists' do get things wrong occasionally.
Whatever the problem turns out to be, it will be something that can be rectified, possibly something quite simple - so no call to "wash your hands of the car" in a fit of pique
The important thing is to get it put right before it causes another HGF (assuming it hasn't overheated to the point of doing damage already).
To be honest, it wouldn't be the first time we have seen a post claiming work was done by an experience MG Rover specialist, who then turns out to be nothing of the sort (although we haven't seen a certain one in particular mentioned for quite a long time now!).