I don't want to go into a long answer - but assuming that contract law in the UK is similar to the US - even if a minor "certifies" that she/he is over 18 - if not, the contract is still invalid "on its face".
Simply put, a minor cannot enter into a contract whatsoever.
Yes, but in above example model confirm she/he is over 18 and thus have rights to sign contract. Tah is purpose of sentence to make model responsabile for claimins she/he is over 18 and have rights to sign contracts.
If you exclude that sentence and model sign contract, what then? If model sign contract with this sentence, and after that happens model was under 18, than model lied and photographer is less or not at all responsabile.
If model sign contract without that sentence, than model can tell photographer didn't asked for hers/his age, and then photographer is responsabile if model is under 18. That sentence put responsability for legal age claiming on model, not photographer. That is purpose of that sentence.
I don't know but, I would ask model to show ID card, or passport, and beside models signature I would write number of ID card or passport.
For example:
Test of contract/agreement
model signature
______________
Number of ID card/passport
_____________________
Without that I would assume model is under 18.
Of course if model in start tells she/he is under 18, than I would make all arangements with one or both parents AND model together. And parents would sign contract. Or legal guardians if there are no parents.
This is important in situation for example: you made photographs, published them, had exhibitions, etc... Then after some time it is found yours model was under 18 in time of photographing. In that situation you may be forbidden to use those photographs further, but you are not responsabile for previous use, because model granted she/he is over 18, and model is responabile for not telling the truth. If there is no sentence like above, you are responsabile because not checking how old was your model in time of signing the agreement, that is time of photographing.
Regards.