Based on your updates having read the full thread, honestly, i think they haven't done anything wrong.
The event you were attending was free, so they weren't expecting a cost for attending.
They hadn't booked accomodation, it is feasible, even though you personally think it was too far for a day trip, that they intended to save money for accomodation by not staying anywhere, and may have either left early, or driven home afterwards anyway. What you deem too far they may have been willing/felt able to do, especially if one slept for half the trip then took over driving etc, to attend without having to find and pay accomodation.
Your husband didn't tell them a price, or say they needed to pay, and used terms that the room was now going spare, giving the impression its paid for, and can't get a refund for. It could be seen, as you say it's his oldest friend, that they genuinely believed it was being offered for free as you knew they were attending and not already staying over somewhere and room was now going spare.
With regards to breakfast, YOU wanted to eat at a specific place you'd been to before, and invited them to come along. Some people take invites like that to mean inviter is paying, similar to when someone asks you out on a date etc. Plus, the place you picked was expensive, £35 for breakfast for 2 is ridiculous, i'd only just pay that for an evening dinner for 2, and breakfast is always cheaper, you could get an eat as much as you like buffet breakfast for way less. Especially given that they seemed to not have been planning a stay over anyway, and thought the room was a gift, and being oldest friend, they may have thought this invite was an extension of the original offer they thought was free.
You knew what this person is like, that they have history that lead you to ask your husband to explicitly state intention for them to pay for room when offering, so you knew there was a need to state clear intentions to this person, and yet in case of husband with room and your breakfast invite you didn't do this. Maybe breakfast was him just milking it (GF paid for them, since it was him husband invited on trip she may well have been told by friend it was a free room and she is probably mortified) but the fact they got embarrassed when you asked for the money, i would assume honestly they may have trouble affording it. Even if they have good jobs, they may have debt, or be bad at budgeting and just hadnt factored an £80 bill (or the £35 for breakfast) and their free event plans just suddenly ended in a bill over £100.
I certainly couldn't just find that to hand over without at least a months planning of where to cut back other places. Either they will pay you when they have it, or you just have to write it off as your mistake.