In fairness to BT, yes utilities have statutory powers to install equipment, even against the landowner's permission. But they still have to pay a reasonable wayleave fee and, don't ask me why, but BT's wayleave fees seem to be very generous in comparison to (say) power companies. I have an electricity pole in my garden too, so I speak from experience.
Plus, if the property owner subsequently decides he had other plans for that bit of his garden, like maybe installing a nice bit of block paving, or building a shed, then the utility company ( given reasonable notice) usually has to relocate the services at their own cost. That does depend on the legalistic wording of the wayleave, mind you.
As suggested by Eric though, maybe some bits of this thread wants splitting off?