Re: @Doctor
"Secondly, you can't refuse the service if the checkbox isn't ticked."
Why not? There's plenty of businesses where if you don't sign their contract they won't do business with you, what's different in this case?
From the ICO's site:
Consent under the GDPR must be a freely given, specific, informed and unambiguous indication of the individual’s wishes. There must be some form of clear affirmative action – or in other words, a positive opt-in – consent cannot be inferred from silence, pre-ticked boxes or inactivity. Consent must also be separate from other terms and conditions, and you will need to provide simple ways for people to withdraw consent.
I doubt refusing to provide the service if consent isn't given constitutes "freely". Also not "separate from other T&Cs" - again you can sign the contract but refuse consent. I think anyone trying to make consent a condition for providing a service will find themselves answering to the ICO or equivalent in other jurisdictions.