A search of the forum will find a few discussions which may be of use.
In general, NRW (nee CCW, nee NCCW) have kittens whenever anyone considers panning in a river as the disturbed sediment will damage some mythical little weevil. Merely asking them for advice is probably going to result in river bailiffs following you around for months.
I wouldn't like to quote the phrase 'ah, but it's the getting caught that counts', but I'd tend to be more concerned about landowners if I were you...
(If I were of the argumentative type, I might wonder how it is that someone gold panning can cause measurable environmental damage, particularly when compared to a river being in spate after winter rainfall - but I won't...)
Hello again darkness, my old friend...