One of the regulars here BruceTF135 invented a cracking low coolant alarm that just needs a sensor to be glued to the coolant bottle. It plugs into the existing wiring at the back without the need for any cutting into the loom . Ive had one in use for some while and have recently bought a 2nd one to fit to the other car Im doing now.
Other options are available including one that replicates the type fitted to later MGs that has a coolant tank with an internal float.