According to Oxford Dictionaries, "Fruit chips were typically dried or otherwise preserved, while potato chips were fried. Britain retained this usage, especially when referring to fish and chips." So, they just kept referring to fried potatoes as "chips." And, fwiw, American-style potato chips were introduced to the UK in the 1920s. To avoid confusion with their term “chip potatoes” they were called "potato crisps" or just "crisps."