Just copied this off a UK wesite called gardenlaw.co.uk
Protecting against Trespassers
Traditionally owners have put deterrents to trespassers on their wall or fence and if it is a party wall just on their side. You will often have seen spikes or broken glass or barbed wire on the top of walls.
However, if the wall/fence borders the public highway, then the local authority has powers to protect the public by having the deterrent removed if it thinks it could cause a danger to highway users. If the deterrents causes a nuisance or by your negligence injures a passer by, then they may be able to sue you. Err on the side of caution because if the deterrent is considered to be excessive you could open yourself up to a criminal prosecution for an injury caused.