Do you just want to keep British species or are you going tropical too?
You certainly need a set of tanks showing different sections of a British river - headwater stream with bullheads & grayling, trout reach with trout and minnows, slower reach with perch & roach etc and then estuary with mullet & flounders.
Marine tanks include a sea grass bed with seahorse & pipefish, rockpools with blennies & anemones, a big tank for conger eels, dogfish, turbot, pollock etc, a wrasse tank (for colours) and an octopus tank.
Hope this helps
Alan