Posting fish

This option has become quite popular over the last few years, however their are several things to remember.
Royal Mail only allow live fish on their Special Delivery service and it's stated that these are only for fish fry and eggs.
The reason for this was to cash in on fish farms in the UK which used to post eggs and fry between each other and around the world.

Living_creatures

Bees, Leeches, Pupae and Chrysalides, Caterpillars, Lugworms, Rag worms, Earthworms, Maggots, Silkworms, Fish fry and Fish eggs, Mealworms, Crickets, certain Parasites and destroyers of Noxious Pests, and some other insects sent between recognised institutions are allowed. You must use boxes that protect both the creatures and Royal Mail staff from harm. Use First Class as the minimum service and clearly label the package as "URGENT - LIVING CREATURES". Mark the sender's name and address on the outer wrapping.

(From Royal Mail)

So anyone posting young fish and eggs your legally fine to do so.