Thank you so much all, these responses are a mixed bag and I appreciate the advice and perspectives!
Day out (I shouldn't have called it that, it was 3 hours altogether) has been and gone, and all was fine, everyone (including dog) happy and back now.
What I did was compromised. Early on, we left her and went for a walk but nearby, listened to see if she barked/howled and nothing. Took her for a decent but not too much for her walk this morning, left her at the AirB&B (with TV and fan on) for the talk and guidance bit, and then we were all left to look around, so I went back earlier than my friend.
I gauged it on what she was like. After our walk this morning she was tired and sleepy. Not a muff out of her when we left. When I returned (maybe just under 2 hours later) she wasn't even that fussy over me coming in, didn't come down to greet me, and was still in the same place she had been when we left. Seemingly very unbothered.
@AlcoholicAntibiotic The terrain where we walked was very uneven, lots of rocks etc which I don't think her joints would've liked albeit she'd have done it, plus I wouldn't have been able to do much of the looking around as she'd have been just keen to get in the sea! So it was the right thing to do.
Once back I stayed with her for a bit and then walked her back to meet my friend on her way back, stopped and had a glass of wine and so dog could have a rest, then back.
She's absolutely fine 
As @WeddingInvitation says, dogs do make it quite clear when they're not interested in going out and would be happier left to rest and mine very much did this morning. And it wasn't for long. It's only 19 degrees here, warm but not hot.
@Tinywhitebutterfly you were right, she was fine. She's an intelligent breed, I think although she is anxious and clingy she knew what she needed and had cottoned on to the 'this is our home for a few days she they're coming back' so she was fine, I was more the worried/clingy one here!
@parachutegirl I was a little worried but had relaxed a bit and was quite glad when my self-allocated time had come to go back to her! But I told myself it was so unlikely anything untoward could happen, and I'd seen how relaxed she was this morning. She was very unbothered when I got back, walking her was a good idea. And no, unfortunately she's over 6stone! If she was a pick-up-able dog that would've made my decision easily, I'd have just carried her about.
@BillieWiper 'It'.
I knew she wouldn't do anything like that unless she was ill, which she wasn't, and I am more likely to break something than she is if I am honest clumsy as hell
I think some folk do catastrophise, it is always possible that any AirB&B guest (dog or otherwise) could have an accident or break something. I would just clean it up/replace anything damaged or pay for it, job done. These things come with risks, like anything else.
Thank you again for all input, I was unduly worried about this.