For what it's worth, I never bother to open the engine block drain port except when the engine block is out of the car and stripped while going to the machine shop. If you try draining the engine block through the standard drain cock (after one thorough cleaning), you will likely find the drain port clogged up again one year later (sometimes as little as 6 months), especially if you use plain water for coolant. See separate article about Crud in the Water Jacket. People using "permanent" antifreeze will not drain it more often than once every two years (or longer). R&R of a plug is easier than R&R of a drain cock, and hardly any more work than opening and closing a drain cock without breaking it (even if it did drain).

I don' t know why anyone would ever open that drain port anyway. Perhaps it was originally thought that it might be easier to drain from the block than to drain from bottom of the radiator, but it certainly does not work out that way in the real world. Many years ago it was common to use plain water in the cooling system in mild (no-freeze) climates. Even in the mid 50's when these cars were in production it was still common to use alcohol for non-freeze protection in cold climates. Alcohol would boil at summer temperatures, so each spring it had to be removed and replaced with water, making you do the process twice a year. Either fluid would lead to considerable rust in the engine block, but perhaps a few months of alcohol solvent soak plus twice a year flushing might keep it clean enough for the drain cock to actually work. This was common practice when I was a kid on a farm in the 50's, but I have never tried it myself. By the mid 60's most everyone (including myself) was using "permanent" antifreeze (ethylene-glycol) and leaving it in for at least two years at a stretch.

The easy way to flush the cooling system is to disconnect the primary heater hose and connect a garden hose on the open heater valve. If you intend to do this more than once you can install a flushing "T" in the primary heater hose. If you don't have a heater you can install a pipe coupling in the port on the head. Remove radiator cap and flow water in reverse through engine block until it comes out clear. Drain radiator (disconnect lower hose if no drain tap on radiator). It matters not that you may leave a quart or two of water in the engine block. The MGs have 6-quart capacity cooling system (maybe a quart more with the heater), so after flushing and draining the radiator, install 3 to 3-1/2 quarts of permanent antifreeze before topping off with water. (This will not work with pre-mixed antifreeze, where you would also have to drain the bock to remove the plain water).