What part of "wheeled" is confusing to you?
Rollator is one of the listed wheeled options. Non-wheeled mobility aids ("cane, walker, crutches, etc.") have their own category.
There isn't really much difference between a standard walker, and one that has two wheels, so far as a mobility aid goes. A four wheeled rollator often adds a seat, some are designed to be also used as a transport wheelchair.After you check the box on the booking page for "Disabled" and then click "Find Trains", a dialog comes up that asks for your type of disability. I think Walker is one of the options. Try a fake booking for an LD train to see if the Accessible Bedroom shows up under Rooms.