I cant really help with the law aspect of this post, but i can give some tips on how to collect plants
with stem plants (hornwort, anacharis etc.) just snip off a healthy stem, no pulling out of the ground required
Getting plants like water lilies and vallisnera is trickier, you have to carefully dig up as much of the bulb and roots as possible with minimal damage
transport aquatic plants without any soil in individual water-filled (halfway), buckets separated by species, and when in the pond/aquarium watch carefully for things like parasites and insect hitchhikers (though the latter can be beneficial),
really though getting water lilies from a pond or lake is a tricky, long process that i have never delved into, mainly because most of our local lilies are the invasive and less then attractive Nuphar species