I don't know every official dealer countrywide, but you can check here http://www.yamaha-motor.co.uk/dealerlocator/?mm4=GB
or if unsure you can call Yamaha UK Motors to ask confirmation on this number:

Useful Contacts
Listed here are some contact details that may be of assistance

Yamaha Motor UK Headquarters: - 01932 358000
Yamaha Brochure Hotline: - 01932 358121
Yamaha Assist (RAC breakdown assistance): - 0800 246 855

Yamaha Off Road School: - 01543 271675
Yamaha Off Road Experience: - 01686 413324
Datatag Id Ltd: - 0845 0700 440

If you spot a problem or error on this site, please feel free to let us know at:
incident@yamaha-motor.co.uk This e-mail address cannot accept general enquiries. Either contact your Yamaha Dealer or call us on 01932 358000.