Its the airlines. However I've flown with quite a few in the last 11 years and they all have the same rule and it still applies. I've flown Virgin, BA and Easyjet and First Choice in the last 3 or 4 years and they are all the same. I do ring the airlines though, always before I go as I want to tell them my daughter has a pump and will be wearing it and she also has a sensor (CGM) and should have it on and is that ok blah blah and now I always ask if I get an extra hand luggage due to medication and they always say, 'yes for medical conditions, one extra handluggage'.
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