Australia has had a lot of livebearers released into its waterways in an attempt to control mosquitos (gambusia, all species of which are now declared noxious) or by irresponsible aquarists who got sick of looking after them. Tilapia (Mozambique mouthbrooder) and other african cichlids have also done a lot of damage. Result being that the quarantine regulations are outrageous, and it's so difficult to get a species cleared for importation that there are some species that are not represented in Aus at all. The Customs people are too good at finding them to attempt to smuggle them, and most Australians are too honest to buy fish they know were brought into the country illegally. I wouldn't, anyway.
I've never even seen most of the species of rarer livebearer (living in a small country town) but the ones that there are are worth fortunes here, over 30 or 40 dollars each. If they're cheap in the UK, you are seriously lucky. Endlers are a bit cheaper, but still about 15 dollars each.