but the holder of a foreign PPL can fly to the 1500 ft minima
3k vis applies to all JAA PPL's. See schedule 8 of the ANO. 1500m is the VFR minima, but only CPL holders or PPL/IMC holders can fly in these conditions under VFR. In both cases the pilot has had training in the use of radio navigation equipment. This would suggest that the CAA recognise that pure visual navigation is impractical in such conditions. Also, the requirement for 1800m vis for take-off and landing for an IMC holder suggests that when you really do need to see something (ie a runway), 1500m is inadequate.
With 1500m vis you have a circle of view of 3k (1.6nm). At 90kts this means you are potentially uncertain of your position every 1 minute if using ground features and dead reckoning. Keeps you busy in the cockpit! Fly over a featureless patch for 1 minute and your guessing (or hoping) really.
If you can't navigate in 3 Km visibility then clearly you are not being taught the correct techniques!!
IMHO the correct technique to navigate in such conditions is to build up experience in better conditions first, then apply said experience along with further training (such as IMC) to do it safely. There is a lot more to flying in poor vis than just navigating.
A GPS is the best way to navigate
Don't agree with that one. It is one of many means to navigate by. The other systems are also available for flying an approach.