MJB: the UK position has substantially changed via the CAA. There is now a permitting process and there was a change in the ANO last January. There is a 400ft AGL height limit - but the operator must remain in visual contact with the UAV and the max distance for this is 500m. Must not fly over congested areas at less than 150m, nor within 50m of any person, vehicle, vessel or structure "not under the control of the aircraft operator". Not within 30m of any person. Must have fail-safe systems. Not allowed in CAS or ATZ without ATC permission.
And all operators must have British Model Flying Association "B" certificate.
There was an incident at Buckingham Palace in September which caused
this to be released.
And your point about number of engines is irrelevant. The most popular models have 6 or 8 electric motors.
Here's a 4-engine one controlled by iPhone and a wifi connection.