Below is the syntax for sp_addmessage. Specify us_english for the parameter @lang and add the message. After this you should be able to add the british version of the same message.
sp_addmessage [ @msgnum = ] msg_id ,
[ @severity = ] severity ,
[ @msgtext = ] 'msg'
[ , [ @lang = ] 'language' ]
[ , [ @with_log = ] 'with_log' ]
[ , [ @replace = ] 'replace' ]