Walkers is the English label for the international brand 'Lays'. Lays are essentially Walkers under a different brand name.
I guess that they've decided to keep the change-hating British public happy as opposed to having a universal product name.
The British market always goes berzerk when a product's name is changed. Remember the riots of 86 when Marathon changed to Snickers?? Or the Bank of England bombing because Opal Fruits wanted to join its European colleges and turn over to Starburst. Or When the Queen went on hunger strike because she was no longer able to clean the royal toilet with Jif liquid, but instead had to use Cif.
I think Lays are making a good choice by keeping Walkers for us.