When you say privacy, can people actually see you from the warehouse, or is it that you can see the warehouse, which is an intrusion?
18 metres is a giant, more than 50 feet, nearly 60'. That's a hangar
Whatever's the MOD using it for? I don't think a few trees are going to make much difference to you seeing it. You can work out, using a bit of trigonometry, how high a hedge planted close to your house would need to be to obscure the new building from your view. The closer to your house, or wherever you don't want to see it from, the shorter the hedge can be.
The quickest growing trees are poplar and willows such as goat willow, which have the advantage of being useable as fire wood once you no longer need them as a screen
For the blackthorn, hateful stuff that it is, you are best to cut it a bit, say a foot above this year's growth, then the same next year, so it stays bushy at the top. I suppose it could get scraggy at the bottom, in which case the hedge would need to be laid, but that's likely to be a good few years down the line, and by then all the other plantings should be quite big.