I think the law you are looking for is 'The Salmon & Freshwater Fisheries Act 1975' (sorry I don't have a link) which outlines illegal fishing methods and the regulations on net sizes from what I can remember.
Nets are not covered by a rod license, you need a special net license, but..... they only seem to be given to fishery keepers and environmental scientists studying fish (and other such people)
Hope this helps you out.
Ed