To decline a FWR you need to satisfy one of the eight statutory reasons - on face value I am not sure you are but that's not really the question you are asking.

If you cannot satisfy one of the 8 reasons for rejecting, then you cannot mitigate this by offering something else (particularly on worst T&Cs). So, first question always must be can we meet one of the eight reasons for refusal.

Of course anyone might accept anything....

In reply to Keith:

I agree with the Keith above. A good answer to the question. Although, I do have a question, about a step that line managers can miss, before moving to consider a rejection. Can we come to a compromise that is workable long-term? When you respond to a flexible working request, you have the following options:
1. Accept
2. Refuse
3. Agree a compromise
4. Agree a trial
5. Agree a trial of a compromise

I would consider the option of a compromise: permanent or trial. The compromise could be: some day shifts, a reduction in hours or a bi-weekly rota which allows him to remain in that role and reduces the impact on the team.

The employee transferring to another role will impact the team. So a refusal looks like a bad option. Also it's difficult to ignore that his wife has cancer and he is asking for more flexibility from the business, another reason to consider a compromise.

It sounds like your reason for refusal is:

"won't be able to reorganise work among existing staff"

But you prevaricate between whether it is merely very difficult or impossible. At least part of the difficulty, you acknowledge, arises from short staffing in the team because the role is "difficult to recruit for". So the first question is whether, if the team were fully staffed, the request would be easier to accommodate. After all, if he leaves the team for the warehouse job, this is only going to exacerbate the short staffing in the shift team and increase pressure on those remaining.

I think his caring responsibilities, whilst important to him and to your sense of compassion, are a red herring to the decision. You are obliged to allow reasonable adjustments (arguably) or risk being accused of indirect disability discrimination. But the whole point of the process of determining whether to agree a FWR is establishing whether the request is reasonable.

My thoughts in this case are that you should support his move to the warehouse, but look hard at what you can do to make the role he's leaving easier to recruit to, with the proviso that, if you can top up the team, you would re-consider his application if he wanted to return to his old job.