My S-plan system with system boiler and unvented DHW tank has only one
zone valve on the DHW primary circuit but surely the safety cutouts built
in to the boiler itself are sufficient as backup safety devices i.e. two
zone valves are not required.

The wording requires the temperature measurement to be local. It
specifically states "if the unit incorporates a boiler the thermal cut-out
may be on the boiler". This suggests that an external boiler that is not
incorporated may not use its cut-out to meet the requirement.

However, I've just examined the Megaflo installation instructions in detail.
It appears that you can use just one motorised valve, if you use S-Plan and
ensure that the zone valve power goes through both the overheat and normal
thermostats. It is only necessary to have two valves if you use 'Y' plan,
where the 3 port valve can't reliably isolate the cylinder. The systems I
have seen have been 'Y' plan, so have needed the additional valve.

So it appears that you can just use the one valve for your S-Plan system.
You just connect one switch wire to permanent live and the other to your
boiler call for heat, in parallel with your heating valve.

Christian.