The helis over Hammersmith going North are probably not on the heliroutes and would be twins.
I don't know what limits ATC use to determine whether a hele is OFF the route. It often is not much though: they have eyes in the backs of their radar heads. I think they listen a lot to the voice on the RT. I suspect they get more concerned when the separation between IFR traffic and the on route approaches the defined separation limit.
As ST already said there is an opt out from normal IFR radar separation standards if 2 heles are on a route, the vis is better than 6K(?) and the pilots agree. However, if one hele is OFF a route then ATC must apply standard separation. You get some odd effects. If a police hele is on scene in the Zone off a heliroute but close to it within standard separation limits, then heles on the route will get held. However, if the police hele moves off scene onto the route, ATC can clear the other hele through. Police pilots will often volunteer to move back on the route if they can to allow this.
The other main exception to standard separation is when ATC can separate traffic visually. This happens in ATZs in the London zone when the TWR controller is visual with both aircraft. That's how heles can do a LHR crossing which is one of life's real buzzes no matter how many times you do it.