I can confirm that if you are wishing to import aquarium plants from a non-EU country then these will need to be accompanied by a Phytosanitary Certificate. Please see the information below which provides links to all of the various documents and information that you will need for this process.
All Plant Health restricted material being imported into the United Kingdom from any non-EU country need to be accompanied by a phytosanitary certificate (plant health certificate) issued by the Plant Health Authority in the country of export. You will also need to register with Fera as an importer of restricted material and pre-notify arrival of the goods. A full list of restricted material can be found in Appendix A of our Plant Health Guide for Importers at:
http://www.fera.defra.gov.uk/plants/publications/documents/importersGuide0909.pdf
A summary of the procedures to obtain plant health clearance are given below:
1. Obtain a phytosanitary certificate (plant health certificate) from the Plant Health authorities in the country of origin. This certificate tells us that the consignment has been inspected by the proper authorities and has been found healthy, free from plant pests and diseases.
2. Pre-notify each import on the electronic PEACH system before arrival. This notification should be at least 4 working hours if brought in by air and in all other cases (including ship) at least 3 working days. You can register with PEACH at:
http://ehmipeach.defra.gov.uk/ (if you need any help registering or entering you information on PEACH you should contact the PEACH helpdesk on 0845 607 3224).
3. Finally, the original phytosanitary certificate should be posted to FERA Plant Health, Room 10GA01, Sand Hutton, York, Y041 1LZ no longer than 3 days after the consignment is landed.
Restricted material will be subject to inspection and documentary/identity checks by Fera on arrival in the UK and there will be charges related to these activities. For details of the charges please see the Fera website at:
http://www.fera.defra.gov.uk/plants/plantHealth/imports/importCharges.cfm