I have two lathes, where I need to cable these up. It's only going to
be on very rare occasions that both will be used at the same time. I
would be happy to make this a rule.

They have 3hp motors which I'm assuming will have a current rating of
rather less than 5amps.

Because the cable run is going to be around 100ft/30m I was going to use
a single 2.5mm2 x 4 SWA Armoured cable to both machines so as to limit
voltage drop during start-up. I have no idea what sort of starting
current I should expect for a DOL motor, ie one without any soft start
capability, although some articles suggest 5-7 times.

I was intending to use a 16A MCB, type B or C. At the moment there is
no RCD protection.

Would this be reasonable?