Hi, butting in from the France forum, but it's a universal issue.
Apart from the best procedure to get your gf and family into Spain, her children's father's permission must presumably be obtained for them to leave the UK. If he gives it, bear in mind that they'll be separated from their family and friends, to live in a foreign country where they presumably don't speak the language. If they're younger than 9-10, they shouldn't have problems in Primary, learning the language, making friends and generally integrating, although, if their mother and yourself can't help with their schoolwork, then they'll need outside help to cope with Spanish Grammar/History/Culture before going up to Secondary School. They could, of course, go to a British/International School, but they wouldn't integrate as well. Bear in mind that, at 18, they might want to return to the UK for further education or for various other reasons....
You'll be taking on your gf's children, please include their future in your research!

They are twin girls and are 3 in September and the father has never met them through his own choice. I have been in Spain for 12 years and I speak Spanish so nothing like getting them accustomed to the lifestyle would worry me but you have just reminded me of a problem a friend had trying to put a child into school the school wanted permission from the child's father for obvious reasons like people running off to another country with children.

This is just things I really need to think about going forward, we are not trying to rush into anything but we both feel that soon enough we are going to want to be together as a family. As time goes on I would seriously consider adopting the girls as my own legally. Maybe that would change my situation 🤔

You don't have to get married, heterosexual couples can have civil partnerships now.

What you need is good old EU freedom of movement and you can regain it by getting an Irish passport. God knows, UK nationals are queuing up for them right now.

The FBR is the Irish foreign births register. If you have an Irish parent, you can apply for an Irish passport immediately but if you have an Irish grandparent, you need to apply to the FBR first and the current waiting time is about a year.

I don't know the exact details of what you would need to do regarding your Spanish residency but you don't need a TIE card if you are an EU national.

Your life will be much easier with an Irish passport and EU freedom of movement.

An Irish or other EU country passport would be more useful for the new partner because then she could just move to Spain (legal stuff for kids aside). It doesn't really change the OP's situation, he's already got residency and Spain allows family reunion for foreign residents.

Having an Irish passport makes a lot of difference actually.

My partner is still British so I'm able to make a direct comparison.

It's a no brainer, if you are eligible for an Irish passport then get one.

If you were to apply for Irish nationality and officially become a dual British/Irish citizenship which is what was suggested above, Spain would recognise you as the nationality of the passport you first showed them for your identity (British). You should be able to change your nationality by updating your details and showing them the other passport but it would probably be confusing for the people working down at the Policia Nacional in your small town.

The EU said that Britons with residency under the WA can have more than one immigration status but perhaps they didn't take into account Spanish bureaucracy when they said that.

If you were to finally get Spanish nationality then that's when your British/Irish citizenship wouldn't be recognised (also your segundo apellido would be your mother's maiden name).

What about Marriage in Gibraltar under Sec 13 of the Gibraltar Marriage Act-is that viable in the circumstances?
https://www.mygibraltar.co.uk/civil-marriages.aspx

Or marriage in the UK?

But getting an Irish passport is a definite if you can, especially if you decide in the future that you want to live in another EU country

Yes my grandad was from Dublin so it will be the FBR route. I think it's worth me going to Dublin to the register office to obtain his birth certificate and start the process.

As for the Mrs I think we will just stick to the 90 day rule for now but the kids will need to start nursery this year so we do need to start thinking seriously.

Thanks for the input everybody it's just one of those awkward situations I suppose and you can't help who you fall in love with hey.

I didn't give a reason for having to provide the father's permission, but it's certainly to prevent one parent from taking children out of the country without the other parent's consent (= kidnapping). Your gf would have to get professional advice, as the father is still legally her children's other parent.
I would say that 4 years is an ideal age to settle in another country. The twins would learn to read and write and learn Spanish history, just like their Spanish peers.... And you could give the British version of the Armada, for instance, when the time comes. I remember having to argue the British side of Napoleon's story when my daughters were at school....

​​​​​​Yo u can order all the Irish certificates online. I had mine sent to my parents' address in the UK and it took just over a week.