Just be careful with fence toppers such as carpet grip, they not only can cause injury to the cats, squirrels, birds etc, they are also illegal under the Highways act if put on a fence less than 2.4m (7ft 10") high:
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Highways Act 1980
Section 164 (Injurious toppings)
The local highway authority has a duty to ensure the safe use of the public highway and this section deals with the problems of injurious toppings on fences and walls that are so low that they could easily injure a person simply walking along the public footway.
Section 164, clause (1) of the Highways Act 1980 states the following:
Power to require removal of barbed wire
Where on land adjoining a highway there is a fence made with barbed wire, or having barbed wire in or on it, and the wire is a nuisance to the highway, a competent authority may by notice served on the occupier of the land require him to abate the nuisance within such time, not being less than one month nor more than 6 months from the date of service of the notice, as may be specified in it.
The term ‘barbed wire’ has been interpreted to mean anything with spikes or jagged projections and this would include any made aggressive topping or addition to a wall or fence including carpet gripper, broken glass, nails and preparatory products such as plastic and metal spikes.
The term ‘nuisance’ has been interpreted to mean something that is likely to cause injury to people and animals using the highway and most highway authorities consider things that can cause injury lower than 2.4m to be a nuisance''