She said that was ok we could just meet up for a day or 2 She’s got zero intention of only meeting you for a day or two. And even if she did, that’s a day or two you’ve lost from your own holiday. She’s trampling on your boundaries because she knows that if she just keeps coming at you, you’ll never say no to her and she’ll end up getting her own way.
Send your friend this message ‘Hi Cheeky Fucker, I’ve been thinking more about our respective holiday arrangements and I’m a bit concerned that you didn’t hear me when I said we were unavailable. Even if we did want company, you like 5 star hotels and excursions to <insert awful place here> while we like hostels and hiking up mountains, so all four of us would be having a miserable time, and we’ve paid too much for that to happen, so we’ll see you when we get back as we’re going incommunicado that week and blocking every single person on our contact list’.
If she argues back against that, or says ‘that’s fine, we’ll just do a day trip or a couple of dinners’ then the only solution is to reply that she’s definitely not hearing you, you said no thank you, you didn’t invite her to your first honeymoon so you definitely don’t want her on your second honeymoon and that this is something you’re willing to fall out over.
I’d also ask how they’ve managed to get travel insurance, tell them you’d be too scared to go to sleep at night if it were you, wondering how you’d cope with a medical emergency in a strange country, a presumably unknown language and really minimal help available from the British Embassy/consulate etc.