TTB;
I cant speak for singles operating commercially over London as I've only ever flown SEs on the heliroutes for transits.
Quite legitimate though for aircraft to hover for commercial reasons - both SE & ME -filming could be for a specific shot/location and the director may only want to do it from the hover.
Network Rail spend a lot of time in the hover and their railways criss cross the heliroutes.
Police have the highest category so take precedent.
In any case - all the operators filming/railways/news etc have to notify LAATC beforehand of their intended taskings as they aren't necessarily following the heliroutes. The heliroutes were designed for transiting the zone. I believe FlyingTV made a case for operating SE helis off heliroutes outside the specified area provided it was in locations where the could alight without endangering 3rd parties.
As for hovering versus orbits to hold - you are correct about being difficult to see on radar but also difficult to see you visually from the tower (the tower wants you visual before crossings). Orbiting introduces a movement which the naked eye can detect (hunter instinct).
Mode S makes radar's life a lot easier nowadays too.