Thermostats are probably the first thing we reach to replace when an engine suffers overheating issues of any nature but seldom is it the resolve. Indeed some remit of the problem may occur for a short period due the fact that the coolant system is bled after such a job, thus freeing it of the air locks that actually (probably) caused the problems.
Testing thermostats in boiling water is a hit and miss affair and is not worth the time it takes. It is better to always fit a new thermostat in all cases. However as stated, alone it is not likely to be the cure or the cause, also removing the thermostat completely may seem to resolve the issue sometimes, however its only prolonging the problem due allowing increased coolant volume flow.
If you have an overheating problem with a Rover V8 its unlikely to be so simple as a thermostat issue, howerver it does happen on older engines that have been stored or unused for prolonged periods. Knowing the many more common issues the thermostat is unlikely to be the end of the matter, however replacing the thermostat is such a low cost and simple exercise there is no problem with trying it first.