Darren is right. When they attend the seminar, if the girl has married in the Philippines, then they will know that her husband had to produce a valid CNI before they would have been allowed to get married in the Philippines.
Likewise, if the girl is a fiancee who has already been granted a fiancee visa, then they will assume that the British Embassy who granted the visa are satisfied that her fiance is single and is free to marry.
This problem only seems to crop up when a fiancee attends the seminar before a she has been granted a fiancee visa.
Iain.