If you walk your dog always carry the means to clear up after your dog. A plastic bag will do the trick or obtain poop scoops from a local pet shop. Remember to take a bag or poop scoop like you'd remember a lead.
It is an offence for a dog owner not to clean up after their dog. Failure to comply can result in a fixed penalty notice of £50 or a fine of £1000 if the matter goes to court.