VFR on top is a standard part of the ICAO definition of VFR, i.e. there's no specific requirement in their standards that you remain in sight of the surface. The same is true in the UK - the Rules of the Air don't prevent VFR flight out of sight of the surface. However if you have a PPL, Schedule 8 of the ANO (which sets out licence privileges) states that unless you have an IMC or an IR you cannot fly out of sight of the surface. Hence if you do have one, you can!
There's no night VFR in the UK. If you want to fly VFR at night in a CTR it would have to be on a Special VFR clearance. And that does require you to remain in sight of the surface even if you do have an IMC or an IR.
Bet you wish you hadn't asked now!
NS