Timothy
For departures, it's entirely down to you as the PIC and whatever your CAA approved company op's manual says, or failing that, the restrictions that may pertain to your individual licence or ratings for the aeroplane.
As ATCOs or the Airport Authority, we cannot stop you departing because the RVR is below a certain level, the decision is yours.
It's always down to you to fly in accordance with your licence, and although we may report the facts if you do look as though you are doing something that is not safe, we can't stop you doing it - I have no desire to be a policeman, but I wonder how many lives would have been saved if we could just delay someone for that critical short while.
What you must bear in mind is that the CAA have issued each airfield in the UK where there is an instrument approach procedeure, with RVR minima for each type of approach. i.e if you want to do an NDB approach at Norwich and the with a CAA approach minima of RVR 900m and the RVR is 1000m, you cannot legally commence the approach.
As an experienced pilot, you will know that the UK weather is very fickle, and what can be CAVOK at the alternate can soon become fog and crap.