The position regarding branches and trees overhanging a boundary is clear in law. The overhanging branches belong to the neighbour whose tree it is, but they may not encroach over the airspace above a neighbouring property. If they encroach, the neighbour is entitled to cut them off in line with the boundary but must hand the cuttings back to the neighbour who owns them.
I suspect the position is less clear if a random visitor cuts the branches of and chucks them back over the boundary but I suspect if push came to shove, it would work out to be the same.