sounds as if they work off the usual electoral registers
http://www.aboutmyvote.co.uk/upcoming-elections-and-referendums/eu-referendum
"How do I check if I'm registered to vote?
Every local authority holds the electoral register for their area. There is no online electoral roll.
If you are not sure whether you are registered to vote, you should contact your local electoral registration office. They will be able to let you know.
To find the contact details for your local electoral registration office, enter your postcode in the Your local area section of our homepage.
If you are a British or Irish citizen (who was born in Northern Ireland) living abroad, you will need to contact the local electoral registration office for the address you were last registered at in the UK."