I first got into collecting here in the states about a year or so ago.
I have been an avid fishermen my whole life but collecting is very different.
what I would do
get your self a good dip net, one like this
http://www.jonahsaquarium.com/netdipnet.htm
for the best results you really have jab the net in the water (often up against structure) and pull it as hard as you can out of the water. (a full day of this is very tiring) Cheap dip nets break almost immediately if you are using them correctly. I have one from the above sight but you could probably call a local fish hatchery and see where they get theirs from I would think that shipping would be a ***** for you. (I replaced my handle with a telescoping fiberglass painters handle, $14 us, works great)
5 gal buckets with battery powered air stones are commonly used for temp holding tanks, until you get the fish home.
I would recommend once you get the net, to kinda scout out your local creeks lakes and streams find out what is in them before you take anything home. Make sure about the legality of everything as well, in the states in certain streams and water ways (trout streams) are off limits to even dip netts. Also certain fish are off limits as well.
I know nothing about British native fish, but because of your climate I would suspect that most fw fish may possibly require cold water (sub tropical temps 70f and lower) and high o2 content. (that is just a guess) I would bet once you found out some fish species you could get more info on them via the net.
I would think that there would be all kinds of interesting minnows, and other fish that would make great aquarium fish. I would like to see what you can catch there. post picts!!!