Short answer:
Yes
Longer answer:
You'd theoetically need the permission of BT Openreach to atach anything to one of their poles. However (as long as you don't damage the pole) failing to get such permission would only be a civil matter (not a criminal one) and BT Openreach could do nothing about it other than require you to remove it.
The Data Protection Act doesn't apply to such cameras, so you should have no other legal problems.
However you'd need to consider the technical problems that you might encounter. The traffic survey firm I work for attaches cameras to lamp posts, telegraph poles and other street furniture (without permission) every day. However we use special cameras, designed to be used outdoors in all weathers. Further, they are powered by hefty batteries which are stored inside big steel boxes which are padlocked to the poles. Video cameras which are designed soloely for amateur domestic use might not be up to the job.
(PS: Although he's not breaking any criminal law, our camera guy has learnt that it's best to set up the cameras when it's dark, or very early in the morning, to avoid problems with people who assume that he's up to no good and phone the police!)