Hi, twiisted! You have chosen a group that may be difficult to acquire in the UK. I believe you need to have a license to possess a sunfish in Scotland, and the only species commonly available in Europe is the pumpkinseed, Lepomis gibbosus. If you can figure out the import laws I'm sure many North American vendors would be happy to sell you sunfish. They tend to be moderately priced, about $5-15.
There are too many sunfish species to go through all of them. This list includes commonly kept species, brief description, and typical size:
Pumpkinseed (Lepomis gibbosus)- olive with dark or reddish spots, blue marks on face, 6-8"
Green Sunfish (Lepomis cyanellus)- basslike shape, variously colored, strong blue marks on face, 6-8"
Bluegill (Lepomis macrochirus)- dark vertical bars, long dark ear tab, 8-10"
Warmouth (Lepomis gulosus)- brown with dark radiating lines on face, 6-8"
Orange-spotted Sunfish (Lepomis humilis)- female drab, male silvery with bright orange spots and orange in fins, 3-4"
Dollar Sunfish (Lepomis marginatus)- olive with bright blue markings on body and face, red belly, 4-5"
Longear Sunfish (Lepomis megalotis)- brightest and most variable species, strongly marked with blue, green, red, or orange, 5-6"
There are also three species of dwarf sunfishes in the genus Enneacanthus (banded, blackbanded, and bluespotted sunfish). These are small (usually under 2.5"), shy, beautifully patterned fish. The much smaller pygmy sunfishes (Elassoma species) are not strictly speaking sunfish; they are under 2" in length and do well in small planted tanks.
Sunfish as a group are hardy fish that do well at room temperature and are not fussy about pH, hardness, etc. Most Lepomis species are aggressive with their own kind and other fishes, similar to many American cichlids. Decor that breaks up lines-of-sight and "rooms" helps allow multiple fish to coexist. Unlike cichlids, they do not pair up. If you keep a mixed-sex group it is better to have two or more females for every male. If you just want fish for display, males are usually brighter; however, vendors usually sell unsexed juvenile fish.
Orange-spotted Sunfish and the Enneacanthus species are more peaceful and shy, and prefer lots of cover.
Sunfish are generalized predators. They do well on meaty pellets, seafood, earthworms, etc.; a diet suitable for an oscar or similar cichlid would also be appropriate for sunfish.