UK - low voltage from mains cable?
There are many applications in home wiring where part of a circuit requires 2 wires travel somewhere.  However, as it works out, they are not hot and neutral - they are whatever and whatever - switched-hot and travelers, hot and switched-hot for a switch loop, you name it.  You as an electrician are expected to know of the existence of such arrangements.  They don't make special cables for these.  You use the exact same commodity off-the-shelf brown/blue T&amp;E that you use for everything else.  And you, as an electrician, are expected to recognize one of these odd arrangements when  you stumble upon it. 

If you really want to iterate on this question, you can keep following wires and testing, and you will eventually discover what this box is associated with and what it does.  However that is orthogonal to your goal, and I strongly recommend against trying to learn electrical by tinkering; this results in a very "holey" education (as does searching Google).  Books provide a far more complete education far more efficiently.