Prices vary greatly on size, type of tree, whether cut or rooted. I think that £40 even for a cut non-drop variety at 5ft is a ridiculous price. Probably not for a potted, rooted tree though, because conifers are not generally cheap whatever their variety at 3ft tall.
Find a tree from somewhere that grows their own and cut them frequently. Never buy a tree that has been wedged into a block of wood as a stand, it will die quickly and be a waste of money. You need to be able to water a cut tree to keep it alive and hold on to it's needles, in the same way as you would a vase of flowers. I'd go halfway through December to get a tree.
Get a special water holding stand, or a big watertight tub with gravel which will hold the tree well and when filled with water as well gives a good heavy and steady base. Based on personal experience, half fill the tub with gravel, get someone to hold the tree centrally in the tub while you add the rest of the gravel. Do this where you want to situate the tree, as it will be heavy to carry. Then fill with water to the top of the gravel. Check water at least every 2 days and top up when necessary.
Before 'potting up' the tree, cut a section off the base of the trunk to open up the water channels so that it can drink. They self heal after they've been cut down, so once you've done this put it in water fairly soon. Position the tree away from direct heat sources, i.e. not next to a radiator. If you do all this, even a Norway spruce should last until Twelfth Night.