Ok, so she loves him and then he when pushed says he 'has feelings' for her... but still wants to make it work with you. Right. So I wouldn't really pay much attention to what he says about his feelings - he is most likely trying to not look like a complete scumbag 'No, don't give a shit about her, but a fuck in the kitchen, well I wasn't going to turn that down...' Basically, he's not exactly leaving to be with her is he. But who cares. You are going to get rid.
The most important thing: if your aim is to live in your country with your child you need to play a very careful game. DO NOT tell him you want to split. The absolute best option would be to go on the holiday, play him like a violin, tell him you can make it work but now more than ever you HAVE to get back home for a while - a year, whatever, to get away from it all.
Go on the holiday, wait until he's having a lovely time in the sun and tell him your conditions are: we move out here for a year-18 months and I can have a fresh start away from where this all happened and we can work on it. Make it sound as if you'll come back with him if it can't work, but don't commit to ANYTHING like that in written form.
Get out there, get her in school, establish as many ties as you can and keep head down and grit your teeth until you have established residency . Make sure you don't get to even talking about divorce until it's been a year. Of course in the meantime if things are 'not looking great' and he comes back to the uk quite a bit, that woudl be fine...
Eventually you will be able to split and stay where you are with your family support and your DD established in her country of residence.
Any other way won't work - he can and probably will use legal means to force you to return home or not leave, when he realises you will divorce him.