My understanding is that you can only isolate the tank from the rads using a valve if they are on a separate 'circuit' of piping - wrong jargon I am sure - correct me if I am wrong in any case?
I wondered about this myself, in my case I believe I can only heat the tank independently by switching off all rads at the individual valves. That's a lot of rads, but I would probably only do this once in late spring, and change it back in late summer.
I was wondering tho whether it would work out cheaper to just use the immersion, because running the (oil fired condensing) boiler just to heat a tank seems inefficient, as you are getting the boiler up to heat, and you have hot water in the pipe run as far as the tank. Mind you, the price of electric right now compared with oil (at 44.5c a litre the other day) made me think that I am as well off burning a litre of oil a day to heat my water (lots of water from that) as to use 2 units of electricity (roughly the same price) which won't go as far in heating water.
Does that make sense?