As I'm sure you know, there's a drain-valve on the starboard side of the M25 block, low.
I put a length of hose onto that (5/16" ID, IIRC) and drained coolant out through that valve into a jug. Opened up the manifold cap to let air into the system.
As Loren notes, it gets a lot of the coolant out, but not all.
When I flushed mine, I drained it, filled it with new coolant, bled air out and ran the motor for a while.... and then drained it, filled it with new, bled it and called it good.
I figure coolant is relatively cheap, and by doing it that way I was (relatively) sure that new coolant made it through all the hoses, the water heater elements, etc.
$.02
Bruce