tetra, most americans tend not to like carp. i think they destroy populations of native fish like bass ect.
me myself, an angler (check me out

), think carp are a stunning fish. nothin like hearing your alarms going off at 4 in the morning, falling out of your chair, stumble down to the bank with no shoes on, and hitting in to one. landing it is another story... or off the top with some floating bread. even better.
same with tench. i like the green better than the golden variant though personally... but thats my opinion. still a stunning fish though.
my mate had perch in a 20gal for a while. they where kinda like oscars, loads of personallity, but sulked and tore up plants ect. they where very shy at first, but quickly learned who fed them. they took worm, maggots and casters from day one. and some plattys and other small tropical fish.
roach would be good, same with rudd and dace. maybe bleak too. all in a tank with loads of flow, and heavily planted, loads of rock and some wood... sound good to you?
i'd steer clear of pike. they get way to big. and barbel too, but barbel may do well in the pond.
as far as im aware, i dont think we need a license in the uk to collect fish. as long as its not from a private lake, i think its alright. but check to make sure. i think game species are off limits though...
if your near the coast (like me) try some native marine fish. get a few corkwing wrasse, or ballen and cuckoo wrasse, and some flatties for your tank. crabs, anemones (beadlet nems are easy to keep, dont need loads of light), starfish, snails, prawns ect. you may even be able to get away with a lesser spotted dogfish if you've always wanted a shark.
maybe some octopus (escape artists), or one of my favourites a cuttlefish, but i think they get too big. like 3ft long, and have really short lives.
i think your fine to take marine species, as you can take home and eat anything you catch in uk waters, exept sturgeon. so i dont see any reason why you cant take your catch home alive instead of dead. as long as specimens are over any size limits.
heres a rock pool tank i had a while back. gettin some more this summer, if the missus will let me...
http://www.monsterfishkeepers.com/forums/showthread.php?t=174218