The rig is designed to US standards, so for a 30 amp service design (US), there is a limit to how many branch circuits can be connected at one time. The fireplace is wired through a circuit that turns it off if the microwave is turned on. It's a pain on a cool morning! There is an extensive thread recently on this (I asked too!).
Also in the UK, you are running 16 amps but at 220-240 volts, so similar amounts of energy as we have with our "30 Amp" connections. It's all at 50 Hz, though, so perhaps the motor has a hard time starting in the right direction? (small AC motors are not my thing)
SF Bay Area
'99 Ford E350 V10
'16 Rockwood 8329SS