It should be the 6th of January, that's the 12th day, i live in the UK like you. They say that it's unlucky to take them down earlier than that or later.
Ours comes down tomorrow on the 6th of January. This year we struck out with our outside tree. We usually cut down one of the evergreens on the property, stake it up and decorate it with lights outdoors. Only this year the tree kept toppling over. Then of course it would snow & ice on the thing. We dug it out three times before we gave up. So no Icabod tree for us this year. We have always dubbed the outside tree "Icabod" after Icabod Crane because it is always a very spindly and skeletal looking thing! Maybe better luck next year.