It is not necessary, under the Protection of Children Act to "prove" that the person depicted was in fact under 18.

"Where the age of the subject of a photograph is uncertain (i.e. where the identity of the subject is unknown), the subject's age shall be determined from the photograph.

Section 2.-(3) provides that a person is to be taken as having been a child at any material time if it appears from the evidence as a whole that he was then under the defined age of a child."

http://en.wikipedia.org/wiki/Protection_of_Children_Act_1978