There can be a lot of misunderstanding about the water bypass on the A series engine. When a later Metro engine is fitted, the bypass feature is often omitted. This is a mistake as the bypass system is important to give the correct water circulation.
The bypass feature allows water to flow around the engine, without passing through the radiator, while the thermostat is shut. Without it, the water will not circulate correctly creating hot spots around the engine which could lead to damage. Some people advocate drilling a couple of small holes in the thermostat plate. This is not the same as having a bypass feature. The water passing through the thermostat still passes through the radiator causing it to take longer to get up to operating temperature. This is bad for the engine. The shorter the time to get to operating temperature, the less wear on the engine components.
Later Mini's lost the little hose between the head and water pump, just like many Metros. However, they still have a bypass feature. In these cases, they have a 'sandwich' plate under the thermostat housing. There is a water outlet on this that allows water to flow out when the thermostat, above it, is shut. It is then fed back into the bottom hose and then into the pump. So, still a bypass, but via a slightly longer route. When fitting an engine without the little bypass hose, it is recommended to use this method.