[maccard]

Also in the UK, plus one to everything above (with the caveat I have no experience in the children area)

For friends and family, the older relatives on both sides I believe just assume we're married.

There is one difference, and that is in default treatment. If I died/was rendered otherwise incapable of communicating my desires, there is now an onus on my partner to prove that we are in the relationship she claims we are. That exists with marriage too except it's "solved" by a marriage certificate.