I refrained until now from commenting on this because it really is a Brit thing. A buttie is a sandwich, a chip buttie is a chip sandwich, a bacon buttie is a rasher sandwich. What the US calls chips we call crisps, what the US call fries we call chips.... same school of English as a rug and a mat, 'cept when it is on a head and it's a wig! Proper chips are twice-fried in lard,however the Belgians say that they should be fried in horse fat, it's an oil boiling-point/temperature thing. Boxty has nothing to do with it, that would not be put in a sandwich... ever.
http://www.irishcultureandcustoms.com/2kitch/rPotatoes.html

Also have a look at Champ, if you are of a culinary bent!
K.