RW, although some cut Christmas trees will go to waste, I hope this information will give you some comfort.
I'm sure in the UK (like the USA) the trees are a planted and cultivated crop. Some farmer, or sylviculturist, planted those trees as a crop, sold them, and fed a family with the proceeds. Like all crops, not everything harvested gets used for the intended purpose.
From a planting trees/conservation standpoint, stands of farmed young Christmas trees provide food, shelter and cover for wildlife. The ground is not tilled, so they conserve soil and minimize erosion. When trees are planted to replace the cut trees, typically they are planted individually in a single shovel slice. Christmas tree farms will never produce a forest (unless the farm closes), but they are sustainable agriculture.
I'm not sure what Dobbie's is selling the £10.00 trees for (that is, what use they think customers want them for). Possibly it was an old sign for last-minute Christmas customers? Most of our "spent" Christmas trees here are used whole for wildlife cover, or sunk whole/weighted in lakes and ponds for fish cover, or are shredded for mulch.
It's good you are concerned about conservation, but I wouldn't be too concerned in this case.
