I like to have isolators on taps and toilet cisterns, mainly at home, it's not a problem on the UK boat. Our shared boat on the mainland has a much more complex heating and domestic water system with lots of isolators and drain valves which get used for winterising as it get's much, much colder than here.
I've never tried to open our drain valve. It's also about 30 years old. I only drained the calorifier once, reasoning that it's bottom is pretty much in contact with the water about 30 cm down and unlikely to freeze. The one occasion I did drain it, I took the PRV off the top and sucked it out.