How Can i reschedule my return flight UK to Pak without paying penality before time
All airlines can provide such a fare, but you have to pay for a higher cost fare type upfront.
While you select your ticket, all airlines will provide different fare options and list the costs for a date change. On that stage select the fare type which suits your flexible return date. Technically that may be free of cost but you will pay that cost upfront.
I doubt you will find any that charges the least and then charges 0 for a date change.