Importation of Animals to the UK
Importation of animals, to preserve bloodlines, enrich bloodlines (i.e. for breeding purposes) and for pet or working reasons appears to be a legitimate exercise. However, it is purely for man's own ends, so the welfare of the animals in transit must be very carefully overseen.
There are regulations governing the crossing of national boundaries and these must be observed. Asking DEFRA in advance of any plans is a wise precaution, to ensure timely compliance with all applicable regulations, relevant to the species involved. It is very bad for the animals in question, if they begin their journey only to be held up or sent back, due to incomplete paperwork or procedures. Of course, there may be quarantine requirements, depending upon country of origin.
Brining animals back to the UK from a holiday requires either Quarantine or a Pet Passport, depending upon country of origin.
Importation of exotic animals, for reasons of profit or vanity, is arguably not justifiable and can represent a welfare disaster (exotic trade) but it is still legal for certain species. The legislation governing which species can or cannot be imported is:
Control of Trade in Endangered Species (Enforcement) Regulations 1997
Another example of unwise importation (while not exactly representing exotic trade) is the Harlequin Ladybird. This was imported to the USA in order to perform biological ‘pest control'. It has since taken over and is invading the UK, too. It is hard to imagine what those who make these mistakes think might happen!
http://www.ladybird-survey.org/harlequin.aspx & http://www.harlequin-survey.org/default.htm