No Op, sorry. I don't know Northampton at all.
But we moved away from London when our DC's were 8, 6, 4, and it has worked perfectly for us. So perfectly, that I really wish we had done it sooner.
You seem to have everything in place for a move. Your fiancé has a job lined up. You have family and friends you'd be moving closer to. You will meet people easily through your daughter, at toddler groups, then nursery and ultimately school. You will be moving to a bigger property, where you may well make friends with neighbours.
I just think it's perfect timing. You won't have to uproot your dd and any future children during school years, because you will already have made the move.
Is the only downside leaving friends behind? That's sad, but any good friends you will keep hold of.
I just don't see any negatives, apart from the upheaval and that it is daunting doing a big move. Although very exciting too.
Does your fiancé have strong opinions either way?