You'll require a countersignature if you look markedly different since your last passport photo. In particular, the guidelines state that it's required if 'you cannot be recognised from your existing passport'.

Do I need a countersignature for my UK passport renewal?

Renewals of adult passports do not need a countersignature. Children passports do require one and even then these days, it can be done online. The passport office will contact the person nominated for the countersignature and ask them some questions.

Does a passport have to have a signature?

Before using a passport, the last most important step a person needs to complete is to sign it. Passport signature is essential as without it the document is not valid.