400 feet agl certainly wasn't a height we often operated at - in fact I can't recall why this would be needed on a day to day basis.
1200 feet was a good compromise for most jobs, especially where built up areas were concerned.
More importantly, is there any evidence to suggest this aircraft (I mean the one we're supposed to be discussing on this thread) was on task, hovering at 400 feet?
BTW, unless things have changed very much since I left the job, UK Police helicopters do not carry out "routine patrols".