"Obviously cheaper than laying underground cables"
In areas where there are no BT tunnels, yes.
My part of London doesn't have poles, all the BT cables are in tunnels under the pavements. It's a simple matter to pull cables into them, and there is probably enough room. There's a new green box being installed a few hundred yards from my house - it's twice the size of the previous one, so something is going on.
If you're not sure what goes on inside these boxes, this video might interest you - once you filter out the excruciatingly corny script.