I have two male friends who, when they split up with their partners, had the mother move 3 hours plus journey away, with small children.
Every situation is different, but I would say this -
It made the fathers deeply, deeply unhappy and frustrated. It really changed the balance of the post break up relationship - it seemed to 'up the stakes? and everything to do with the children became more hostile, formal and legalistic.
it changes the nature of the father's relationship to their child. A father in the same town as his kids can do so many things (nursery and school drop offs, attend school events like plays and parents' evenings, go to football matches and swimming galas etc) - even when it is not 'their week' with the children.
Visits become a much bigger deal when the kids have to travel a long way away - visits seem to be more vulnerable to change, which is very disruptive. Say a DC looks like they are coming down with something, and don't travel on their appointed day. But within 48 hours it seems they are ok. They might miss out on a whole week's visit because of that - whereas when the other parent is forty minutes away, they don't have to book a day off work to come and get them, they can just collect them another evening.
When the kids are school age and a social life with their peers is getting important, it is a wrench taking them 'out of circulation' and making them go to another town to see their other parent.
I appreciate you are saying 'when DD starts school we would have to reconsider things' so that both parents could live near each other. But realistically, what does that mean? You won't move back to London, or anywhere, if you have a good life and are near lots of family for support. Which puts your partner over a barrel.
One of these dad friends, who jumped through every hoop going to keep contact with his kids, once drove 5 hours up to Northumberland to see his DD in her school nativity play. she was delighted to see him - they managed a half hour excited daddy time (the mother knew he was coming, it wasn't a surprise) - then dad drove the 5 hours home again. He was crying for most of the journey. I hope I never split up with my DP (who does) but that story has stayed with me.
I would urge you to look at the bigger picture, whatever you do.