Sorry - just realised I posted on the wrong thread.
But I've had a reply from Tim at UAV Hub:
"The 120m distance is from CAP 1789 under C0 aircraft, whilst this specifically talks about C0 aircraft we imagine it will apply to all sub 250g aircraft. "
The relevant section:
Class C0 - (can be flown in all subcategories) Very small unmanned aircraft, including toys, that:
▪ are less than 250g maximum take-off mass
▪ have a maximum speed of 19m/s (approx. 42.5 mph)
▪ are unable to be flown more than 120m (400ft) from the controlling device
Since the DJI Mini is not technically C0 but rather a legacy class I'm not sure that's correct. You would think there would be clear reference to a max distance in the other sections of the regulations.
Seems as though the
DJI mini 2 might be perfectly timed as a legacy drone whereas future C0 drones will have to be more limited.
Tim also makes the point that all the regs are clear on unaided VLOS being required - and it's certainly arguable that a drone the size of the mini isn't clearly visible beyond a few 100m (though 120 seems short).
Apparently the more recent regs seem to have changed it to height anyway,
See:
Regs changes in the UK - Nov update