This all depends on how vigorously the police want to pursue the case, technically speaking:
- It's a breach of the wireless and telegraphy Act.
- It's a breach of the computer misuse Act.
The individual in question could also be arrested under the data protection act (but this would only be justified if it was a corporate network with a data controller employed there). However, that said, there was a similar case in the UK some time ago and the guy actually went to prison.
So, yeah - it IS illegal. However, I've come accross many open networks and many that are secure but have still managed to hack them due to poor security settings and if I was caught doing this I doubt very much that the police here would be sympathetic considering my experience as I should know better.
Bottom line:
You are accessing and using someone else's resources without permission.