With respect, perhaps you do not fully understand how the system works.
When you open a hot tap, water flows from your hot water cylinder (in the airing cupboard) and it can only flow (with your system) if cold water, from the large tank in the loft, flows into the bottom of the hot cylinder, therebye pushing the hot water out of the top of the hot cylinder and on to the taps. As mentioned by others, turning off the valve, on the pipe from the large cold water tank, will keep the hot water in the cylinder so, no draining required on the cold or hot tanks.
As mentioned, don't plug any vent pipes. In fact, don't plug any pipes. With this valve turned off, any hot water tap can be dismantled and re-washered.
However, the cold taps are likely to be fed directly off the mains so, any work on these taps requires the main stopcock to be turned off.