Its a good idea to swap the lock as it only takes 15 mins, but try and buy one with two rows of drilled depressions as that gives more possible combinations.
Another way is to ADD a UK 5 lever mortice lock to the door, takes about 1.5 hours to fit.. This makes it more difficult for people to buy keys locally.
Wickes used to do one for less than £20 3 years ago, also B&Q.