Copy and paste from bhg481 link
"What should I do with the trimmed branches?
Ironically, even though the branches belong to your neighbour, you cannot simply throw them back over his fence. That could be deemed to be fly tipping of garden waste. Advise your neighbour that you intend to burn them or take them to a recycling centre."
You are supposed to offer them back, if not wanted it's your job to dispose of them.