That is a very good question.
if its a boiler that just has earth, neutral, live and switched live then a 3 pole switch like those used on bathroom fans with a neon and a fuse (if they exist) would be do.
My boiler is five wires, earth, permanent neutral, permanent live, orange and grey wires.
The grey and orange wires just go to all the paralleled up microswitches in the zone valve.
So in theory a simple 2 pole fused neon switch would suffice. However, as there is permanent live and permanent neutral to the control wiring centre, in theory, if a thermostat starts calling for heat, a zone valve will be energised and open.
So there is a theoretical risk that live mains could contact upon the zone valve microswitches, and hence make ornage and/or grey live in the boiler....
At least the two zone valves have un-pluggable cables so power can be cut to the zone valves in an emergency.
SO I suppose I could move the isolator switch from airing cupboard to the kitchen where the boiler is and power the wiring centre from the boiler end.