I was told by the CAA many years ago "do it low, do it fast, do it once and don't come back".
Always works for me - keeping fast allows you to climb it the doink fails or you hit a bird - as well as keeping eyes off the reg.
Rule of thumb - if you can see the reg with the naked eye, it is about 500' away (150m). Take a pic with a 35mm lens (no zoom) and if the reg is visable, the pilot will have some explaining to do.
Good in a Super Cub over naturist sites.
Look up Rule 5 of the ANO.
If they are coming into land or doing a PFL, you can't touch them really. A PFL has to be realistic and that can only occur by flying below 500'.
http://www.caa.co.uk/docs/503/Annual...to31052009.pdf
http://www.oatmedia.com/Patches/AirLawAmendment.pdf