There is no such thing as night VFR in the UK.
In class D airspace you can fly SVFR until you leave a control zone.
Outside a control zone and indeed outside controlled airspace you legally have to fly IFR.
From what I understand you need the night rating to allow you to fly at night in VMC conditions. Obviously if you end up in bad wx or cloud then you need your IMC rating!!!
One of the considerations when flying IFR is that you should follow the quadrantal rule. Other than that the sky is yours!!!
