If it helps, this once happened to me in Kazakhstan, I needed to exit through immigration to re-check my bags. However, I didn’t have a visa and couldn’t get one on arrival. My understanding is that this is fairly common, and an airport staff member I spoke to was VERY helpful, went out, found my bag and re-checked it for me. I picked it up again happily in Kyrgyzstan.
I figure if they can do that in Kazakhstan, Britain should be able to help too. I imagine you’re not the first ever (or even that day) to run into this situation, and if your husband is able to exit with them to help identify the bag, even better.
In Manila, Phillipines I was actually taken through immigration without a visa (under armed guard) to identify my bag for them to recheck it (they claimed they do this for every Cebu airlines transit passenger?!!), so each airport has a way for this, in my experience.
Technically he is allowed to only check in his own luggage. So there is the possibility that if there is a limit on how many checked bags a person can have, he might have to pay excess baggage since all bags would be under his name.
Another issue that may effect your plan would be document verification. His documents would be verified when he drops the bags, but you would not be present which may block the check in process since it is a single booking.
But both of these are just possibilities, not etched in stone. A lot depends on who is handling check in when your husband drops off the bags.
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