Personally I would have thought it would be easier for you to obtain marriage certificates on-line via
http://www.gro.gov.uk/gro/content/If you want to obtain marriage certificates from the Local Registrar, you'll probably have to write a request, visit the Local Registry Office, pay cash, unless you have a Pounds Sterling checking account.

plead with them to send them abroad for you, which means the staff have to go to the local post office, que (we are good at that here) get your envelope weighed and obtain the appropriate stamps, and out of the goodness of their hearts pop them into the mail box for you.
Wendi