Not sure it's a really an offence unless the cost and effort of clean up means it falls within criminal damage or harassment if it is the same oddball bringing a dog round to shit on your property.
Plenty of different rules for cats and dogs though, cats are allowed to have free reign to roam but it's an offence to not have your dog under control.
If you see a dog in front of your car the animals Act 1971 obliges you to take all reasonable steps to avoid hitting it and preventing knock-on effects of causing risk to life or damage to persons and property, although you don't need to take the same action to avoid hitting a cat.