Nichola, if its a replacement passport (ie the old one is running out) then you don't need the signature of authentication if the the person is clearly recognisable.
I have just renewed my wife's passport (10 year one) and sons (5 year one) and had neither photos signed. My son was 10 on his last one and is now 15. we had no problems.
Its interesting looking down the list of people eligible to sign the photo's the website says Countersignatories must work in a recognised profession or be ‘a person of good standing in their community’
Then lists a whole host of professions, it should not be hard to find a signatory if needed.