AFAIK, trespassing per-se is usually not a crime in England. It is typically considered civil matter. It gets more complicated I think if you get told to leave and resist. And of course trespassing with intent to steal is a crime.
I can understand land, but isn't entering an actual building/home/garage without permission a crime even if it was unlocked? it seems like it should be, but my view is tainted by US law which is overly complicated and varies state to state about severity, but I'm pretty sure it's against the law in every state.