Yes, whether you have your device rooted or not, in itself, is not a legal issue in Britain at all.
If you have a rooted device (such as a phone or tablet), it is possible to use some "naughty" apps for hacking networks that wouldn't otherwise work, but as with a sharp knife or car with a speed limiter removed, what you do with it is the important part.
We use the term Jail Break in our adverts because although it is a term properly used to refer to Apple hardware, lots of potential customers still search for this term when looking at Android devices.