Try https://www.google.co.uk/search?q=trike+building+reliant+&hl=en&ie=ISO-8859-1 for a good start on the subject, some good links there.
Otherwise you’ll need SVA (Single Vehicle Approval) so have a look at the government website on https://www.via.gov.uk/vehicle_testing/sva/sva.htm
Hope this helps, let us know when you start it.