Do I put this qualification on my UCAS? Can my referee just leave it as pending?

You need to declare all attempted qualifications including any results to date. Any pending results should be labelled as pending.
UCAS make it very clear you can't pick and choose which qualifications you lost and doing so can lead to your qualification being flagged as fraudulent and a lot of bad outcomes from that.
Moreover the deceit is entirely pointless because if the apprenticeship is as you say unrelated to the course you're applying to it's unlikely to be a consideration (or a serious one at least) in your application anyway.
Just inform your referee you're currently doing this qualification, and they will just note you are completing it with another provider and they aren't comment on it further in their reference.

You need to declare all attempted qualifications including any results to date. Any pending results should be labelled as pending.
UCAS make it very clear you can't pick and choose which qualifications you lost and doing so can lead to your qualification being flagged as fraudulent and a lot of bad outcomes from that.
Moreover the deceit is entirely pointless because if the apprenticeship is as you say unrelated to the course you're applying to it's unlikely to be a consideration (or a serious one at least) in your application anyway.
Just inform your referee you're currently doing this qualification, and they will just note you are completing it with another provider and they aren't comment on it further in their reference.

UCAS states 'Your referee will be asked to add predicted grades for any pending qualifications when adding your reference.' this is one of the only things that worried me. Will it not be an issue if I'm not giving predicted grades for this qualification based upon this? Or will the referee saying they can't comment on it further as it is completed with another provider suffice. Thanks so much.