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I haven't flown LCCs out of LHR but in my experience with BA, BMI, UA and AA they are much more concerned with size than weight. I have had bags weighed (and forced to check) in SYD and BKK but with more than 5 times a year (probably 8 or 10 a year) for years I have never had a bag weighed at LHR although forced to try the sizer multiple times.