Presumably where the coil exits the hot water cylinder is a return -
would that join the CH return and would it therefore do to connect
the loft radiator return to the hot water return?

Yes. Perfect for it, provided it is run in 22mm.

Would it work to tap into the main with a simple T (and same again at
the return)? Ie would enough water flow up 2.5m in this situation, or
would it require a motorised valve to shut off the "easy" route of
the existing CH piping?

Making the water flow down all the lines when the zone valves are all open
is called balancing. You will need to balance the entire system after it is
all plumbed in.

What about a second 3-port valve installed on the CH side of the
existing 3-port valve? This could split the CH water into two
circuits. If controlled by a thermostat in the loft, then this could
switch the CH hot water between the circuits, favouring the loft
first.

Why do you want to favour the loft? A properly balanced system will get all
the radiators in the house hot fast. To subzone, I'd recommend replacing the
3 port valve with multiple 2 port valves.

If you must keep the 3 port valve (i.e. it is a diverter and you like HWC
priority), subzone just the heating circuit with an extra two 2 port valves.
Wiring then gets slightly more complicated as it is non-standard.

Christian.