Unsecured wireless networks are all over the UK, but using them without permission can see you being charged with 'dishonestly obtaining electronic communications services with intent to avoid payment'.
Two people in Redditch have been cautioned over using people's wi-fi connections without permission. In one case the person was arrested after neighbours reported a man inside his car using a laptop. It was early morning and he had put cardboard up at the car windows.
What many people do not realise is that their wireless network is very likely to be accessible and unprotected, which can mean people could borrow their broadband connection. If they used that connection to hack another site or to perform some other illegal activity, the onus would be on you to prove it was not you doing it. Imagine the scenario of someone accessing child pornography sites and the police then tracing it back to your address. They would seize all computer hardware and arrest you; the process of showing you had not visited those sites would be a long forensic search of all your computer hardware.
So what should you do to secure your wireless network? Firstly, you must enable encryption- the preferred methods are WPA2 and WPA, but remember when generating the key to NOT use a phrase that can be easily guessed, such as family names, addresses etc. WEP encryption which comes in two types 128bit and 64bit does offer some protection from the casual passer-by, but tools are available to let people break this in a relatively short space of time. Of course, the best protection if going away for a long time is to turn off the wireless router completely. Wikipedia provides a longer article on wireless security and people are free to ask questions about this on our forums.
A final thought for all those road warriors out there. If parked in your car using your 3G datacard and Streetmap or Google maps to find where you are heading, how suspicious do you look? Would the police believe you were using your 3G card as opposed to an unsecured wi-fi network that you did not even know was in the area?