Advice on single parent taking kids on hols

[Sparklezz]

Just looking for information about taking my kids on holiday.
My ex voluntarily sees the children once a week unless he decides he doesn't want.
I am a bit confused as to whether I would need his permission to take them on holiday. We were never married and have no court order on visitation or maintenance. He walked out on us and gave up being involved in their lives apart from the few hours he sees them, his choice. I have full parental responsibility in.
So, would I still need his permission?
They have his surname and I know this may cause an issue so taking birth cert would solve this. Is there some solid info and advice you could give me.

I have a letter from my ex to take her out the country it's the only contact I have once a year, but there is also an order he can sign which allows you to travel with your dc for up to 4 weeks a year. The latter you need a solicitor to draft and for him to agree to.

thank you for the response makes things clearer for me.

Is he not on the birth certificate? If not and he doesn't have PR then yes you don't need his agreement.. However it is worth taking birth certificate if not due to surname