I've no idea of the UK regulations, but Australia introduced standards for harnesses a few years ago, both for rescue harness and for crewman's harness. Companies have approvals ("Permissions", as they are now called) against not wearing seat belts when using a harness, but they will always stipulate that if a seat belt is worn then the harness must be disconnected: no point in getting caught trying to release both restraints after a bingle!
Our crewman/cameraman's harnesses have to be made to an approved standard, and are usually modified parachute harnesses with the quick release going to a 3 ring arrangement which will detach the securing strap from the back of the harness. A web cutter is essential as an alternative release, usually attached to one of the shoulder straps of the harness. Just as important is the hard point for attaching the harness to the airframe: seat belt anchorage points often do not have the lateral strength to safely restrain the crewman in an emergency. Bell have anchorage points for 206's which bolt to the vertical surface below the seats, other types need to be checked for an approved point.
We also have quick release straps to secure the camera to the harness, a further safety issue which can be overlooked. Then the still photographer changes his lens and it slips out of his hand: but you can only allow for so much, unfortunately!!
There was a thread a while back,
Lap belt vs harness which had some further discussion.