The Ornamental Aquatic Trade Association is the main UK body for the ornamental aquatic trade.
The main OATA web site can be found by clicking the image above and below are some further, more direct links to alien invasive species on the same site.
The OATA position statement on alien invasive species can be found at http://www.ornamentalfish.org/aquanautfeatures/aliens.php
Some specific information, mainly aimed at retail customers can be seen on the OATA conservation page at http://www.ornamentalfish.org/aquanautconservation/conservation.php
You should expect any retailer which is a member of OATA to display this material in their shops and to provide this information to customers when they buy fish or plants for pond use. There is specific guidance on: The dangers of releasing aquarium and pond plants or fishes into the environment at http://www.ornamentalfish.org/aquanautconservation/mustnever.php
OATA fish bags are printed with a warning about not releasing fish into the wild. There is also a poster for point of sale use about not releasing fish into the wild at http://www.ornamentalfish.org/aquanautconservation/petfishbelong.php
There is a poster about the dangers of aquatic plants and recommendations on which plant species should not be sold by retailers at http://www.ornamentalfish.org/aquanautconservation/invasiveplants.php. This poster also gives information on proper disposal of aquatic plants which is normally by composting.