If you like Glenfarclas for its taste and style , then I think you would find Glenlivet and Macallan comparable. WoW has some good editions of both those.
if it must be Glenfarclas or Glendronach, then I don't see any on sale at LHR.
I do like to keep a bottle of Glenfarclas 105 handy. 60% ABV but surprisingly subtle for that.
It's about the only good value at Stavanger duty free.