A competent custom motorcycle shop or upholstery shop or a fabricator who can fit a solo seat frame to the Wing with minor but safe modifications. I've seen custom seats made by Corbin for GL1000's, I'm sure they could make similar for yours. http://www.corbin.com/honda/7579wing.shtml
In the U.S.A. trikes keep the same seat as the motorcycle. Except if someone turns a Harley Davidson Police bike into a trike and leaves the solo seat in place.
The Corbin Gunfighter seat is $329.00 USD + shipping and other charges.
This company is in the USA you may want to contact them: http://sargentcycle.com/custserv.htm
Boy there are some really goofy laws in the UK. Sargent, Mustang and Corbin coem to mind though. Of those 3 I'd say Sargent and Mustang would be the ones I'd call first; Corbin is a tad slow resonding in the past year or so from what I've heard. As an aside, I'd check with some of the local riders near you and see if there is someone that does custom work. I know there's guys in Ireland that do.