I'm sorry that you have this dilemma, these things are always more complex than a simple answer.
I think there are a few things to think about.
Are the children settled and happy there and would a move be very disruptive in a time with a lot of change anyway? When I separated from my partner I decided to stay where I was because it gave the kids stability of friends and schools (but I didn't have the issue of their dad moving too far away so I realise its a bit of a moot point). I did think of moving at some points, (and did when they were older and I remaried) but I felt it was important to give them some anchor of normal day to day life. I know that's not always easy or possible though and kids do adapt with lots of support if you do move
Or do you think they would prefer to move to be closer to their dad so they see both of you regularly? The move is a big transition for them but they have the stability of both parents being around.
Also, some things to think about...
Will you be able to live on your wage (and hopefully with support from ex)?
Would you need to look for another job? And how would that work with solo parenting?
Do you have a support network around you, as you will be parenting solo in your day to day life (with obviously times when they are visiting their dad).
Or do you have any support here in the Uk that would be helpful?
As much as you need to think of the children's best interest, I also think you have to think what you want for your life. It sounds like you'd prefer to stay there and I can imagine mysef feeling resentful if I had to follow my ex back because he decided to make such a big move. This is probably a little dependant on how you feel about him and how amicable or not the separation is and how you feel you will parent whilst not together.
Do you think he will actually parent 50/50 or every weekend if you move? Or could you possibly do this big upheaval and find he is or becomes more absent? And you are left doung all or most in a country you'd rather not be in?
Where will he stay when he comes to visit the kids? Rent somewhere? Or at your home?
There's so much to think about and so many people to keep as happy as they can be in the circumstances (and I very much include you in that) that it may be useful to write down a pros and cons of each decison and also not make one right now. Can you stay where you are for now and if it feels its not right in the near or far future then reassess?
I do wish you well whatever decision you come to.