In the UK, if the door is not locked then it is not breaking and entering, and if nothing is stolen or damaged then it's not burglary.
Which leaves only trespass, which I think is a civil matter.
I remember as kids our doors were never locked, I had no key but would come home from school to an empty house and just walk in. Sometimes a friend would come in, shout to see if I was home and just leave if no answer.
I lived in SF Bay Area (East Bay) in the 1990s, in a house with 6 people and 2 keys. The house was never locked. One day we found kids in the pool in the yard, but they hadn't realised the house was open and had climbed the fence.