Just a few points for the OP...
To be lawful under GDPR laws, consent must be “
freely given, specific, informed and unambiguous”. Employers are therefore best to seek consent for use of images for marketing purposes in a separate contract from the employment contract and to make clear that the
employee gives their consent completely without conditions or fear of repercussions if they do not consent to the use of their image for company marketing campaigns.
The 'specific' part is important and the ICO would likely want to see separate consent forms for specific separate usage (eg. a website article or a specific brochure). A blanket single form would quite possibly be non-compliant with data protection laws.
Generally, consent will not be valid if the employee feels pressured into giving it. It must be a genuine choice and the employee must not suffer any detriment if they decline to take part. So you would have valid grounds to make a complaint to the ICO if they either threatened to withdraw the job offer, or actually did so. (I would expect any legal judgement would extend these rights to potential employees as well as existing ones).
An employee also has a right to privacy under the Human Rights Act 1998. Under Article 8 of the Human Rights Act 1998, individuals have the right to respect for their private and family life, home and correspondence.
More at
https://ico.org.uk/your-data-matters/
(I'm a trained Data Protection Officer btw!)