Thermostats are rated at a given temperature - typically 82 in the case of the MGB - stamped on the bottom of the metal cylinder containing the wax pellet:

Typical Wahler thermostat and its 8 degree or so range from starting to open to fully open, and the 2 or 3 degrees difference between warming up and cooling down to reach a given partially open point. It is the ability to start closing when the temperature has dropped by only a couple of degrees that allows the thermostat to maintain a minimum coolant temperature (in most circumstances) with varying demand on the engine and changes in ambient temperature. The exception is in very cold conditions where surface cooling of the sump, block, and use of the heater may take so much heat out of the coolant it never gets hot enough to open the stat. Image from 'viva-lancia.com'.

A wax stat when cold (left) and hot (right). The metal cylinder containing the wax (7) is in the coolant that is being heated by the block and head i.e. below the valve (3) which is closed. As the wax is heated it expands, squeezing the rubber sleeve (8). At a given temperature this overcomes the pressure of the spring (6) and starts to push the thrust pin (2) out of the rubber sleeve. Because the top of the pin is attached to the bridge (1), which is part of the flange (5) which is clamped between the head and the thermostat cover, this movement actually causes the metal cylinder to move downwards, which pulls the valve (3) downwards. This open the valve and allows coolant under pump pressure to circulate via the radiator and initiate cooling. If the temperature of the coolant starts to drop below a certain point the wax starts to contract, and spring pressure forces the pin back into the rubber sleeve to start closing the valve. Image from Hillier's Fundamentals of Motor Vehicle Technology.

Filling/bleeding arrangements. In the example above there is just a plain hole in the flange, which will allow some circulation of the coolant via the radiator during warm-up and so slow it a little. Below on the left there is an example of a 'jiggle-pin' (circled) in this hole which allows maximum bleeding but zero circulation during warm-up. On the right is a cost-reduced version with no hole in the flange and hence no jiggle-pin, but a very small notch in the edge of the disc valve instead (circled).

The bypass-port in the head. It lies just below the resting place for the thermostat, and you can see that when the original MGA stat's sleeve rose up it would cover this port. Note also the drilled passage inside the port leading down into the block (the faint circular marking inside the port, arrowed). From Bob Muenchausen's 'MGB Engine Cooling' page.

This is the OE Smith's thermostat for the MGA. Note the moving sleeve around the outside of the lower portion. This sleeve blocks off the bypass port in the head once the engine has warmed and the stat has opened. Photo: Neil Cotty, from Bob Muenchausen's 'MGB Engine Cooling' page.

A blanking sleeve from Moss Motors suitable for the MGA, MGB and Sprite/Midget.

Standard MGB stat modified to become a restrictor by cutting the lower part of the frame off and removing the wax pellet with valve, and the thrust pin from the bridge:

At a pinch you can achieve much the same thing by removing the spring (hook one end outside the frame then 'unscrew' it) and manually pushing the valve open:

Typical temperature gradients for the head and the radiator inlet during warm-up, working thermostat fitted. With no stat, or a stuck open one, the two lines would be close and parallel, but flatter taking much longer to reach working temperature. With a stuck closed stat the head line would continue to rise and the rad line would stay very low until the coolant reached boiling point.