Wirelessly posted (Mozilla/5.0 (BB10; Touch) AppleWebKit/537.35+ (KHTML, like Gecko) Version/10.2.1.1925 Mobile Safari/537.35+)
If it's EDI-LON and the fare rules have LCY and LHR as being the same (or however they word it--coterminals?), then I don't see why not, though you'd still be charged whatever change fee applies to your fare.
(I'm not usually in this corner of the forums, but I'm feeling particularly British

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