The CWST may not feed any cold taps. Mine doesn't - all my cold taps are mains fed. The CWST only feeds the hot water cylinder, so in this case bd3cc's response would be correct. In any case, turning off the mains and opening a hot tap would not drain the HW cylinder since the feed to the hot taps comes from the top of the cylinder. Once the CWST is empty, the hot tap will stop.
Steve the plumber is correct You shouldn't be removing the asbestos tank yourself due to the health hazards involved. It needs an expert in asbestos removal.
Ok then so I made an assumption but so have you and so has bd3cc. If mines correct thats the simplest way and doesn't discard any hot water whatsoever which might be a consideration. You'll also note that I said in my second post "via a cold tap that it feeds".
As the OP hasn't come back again noone of us know which is correct.
In any event opening a hot tap won't drain the HW cylinder but it will EMPTY it of hot water the cold feed being at the bottom and the only way the CWST empties is to push all the hot out of the cylinder through the taps. Not that economically sensible.
And the OP didn't say he was removing the tank himself "I'm getting my asbestos tank replaced....." although what you say is one way of interpreting what he wrote.
In any event asbestos cement is only dangerous if it is drilled, cut or broken up. If the tank is in one piece its quite stable. If the thing will go through the roof hatch in one piece then there is no reason why he can't do the replacement himself. Even if it wouldn't he could still do the replacement and move it to an unused part of the roofspace, leave it be and worry about it at another time. No hazard.
Agree my error, opening a hot tap would not empty the cold header tank, so should have suggested a cold tap.
The poster said that he was having the tank removed, which suggests to me that he is not doing the work himself, and from his other comments, is draining the system himself, presumably to make it cheaper, and less time consuming for the proffesional, who can then move on to another job.