I’m devastated, as I really want to go and see me kids enjoy the holiday. I will never be able to take them away like this with all 4 of us again, so it’s really important for me. I don’t want to stop them from seeing their family, but at the same time I can’t sit back while they leave. I just don’t know what to do. Has anyone got any advice?
OP, you are being terribly naive here. Forget your holiday plans. Everything you have said about himthe fact that he is shifty when you ask him why he doesn't want you to come, the fact that he is trying to "punish" you for stuffscreams red flag.
No idea whether the Phils are signed up to the Hague Convention but in any case it may not mean diddly squat. Japan is signed up to this convention, and it doesn't make any difference--they refuse to enforce gross violations. If your ex decided to keep your kids, you would probably be helpless. I am in Japan. I'm sorry to say that I know of lots of cases which had tragic endings.
Your post sounds like you have not properly processed the end of your marriage and are still trying to play happy families in your head and go through the motions of a lovely family holiday. Please don't take this the wrong way, but you need to get smart and get serious about protecting your children.
You need to keep their passports hidden (you say you already have them). Can you send them over to a friend's house? That also goes for any Filipino passports they might have.
Although I don't know the details, sometimes people who have to travel and have lost passports call the relevant authorities and beg for "emergency travel documentation"--I don't know whether the authorities would issue emergency travel documents in this situation if your ex were to try and do this, but I wonder if there is a way, in the UK, to actually notify the border control people and explain that you think your ex might try and get the kids out of the country without your permission, and give your kids' identities.
As others have pointed out, taking kids away to another country and letting extended family look after them is very possible. I have known of several cases in Japan.