I am using a wireless connection from my house to someone else's wireless internet connection.
A few people not so long ago were prosecuted in the UK for doing something like this.
As you don't have his/her permission then yes you are breaking the law.
If I was you I would find out who they are and put a message in the letter box telling them to turn on wireless encryption (or show them how).
And if you keep on "piggybacking" you too could feel the long arm of the law on your shoulder.
Read the above article. You're committing a criminal offence under the Communications Act.
They could also use the Computer Misuse Act if the powers that be felt like it.