Singles can fly in IMC if they are properly equipped and the pilot is trained and licenced. (Maybe not in the UK?) Same at night. As long as you flick the switch to let the engine run on black air instead of white air.
The problem comes when paying passengers want to be inside. The pilot by himself knows and accepts the risks he is taking. The pax expect a perfectly safe flight with a soft landing at the desired spot, but that is not predictable at night or in IMC. Hence the second engine to keep you up and allow you to choose your crash site more carefully.