Stoo_pid has, as usual, proved the inappropriateness of his screen name by providing an excellent answer.
This quote is from the Stockton register office website but it's likely that a similar policy will apply elsewhere:
"The law requires a minimum of two witnesses. They can be family or friends, and although the law doesn't state a minimum age requirement we would ask that the witnesses are at least 18 years old. If you particularly wish to include a witness who is under 18, we would ask that you bring an extra witness who is of the required age."
Chris