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Regarding the car seat issue: I live in London and here, yes, there are stricter rules for car seat usage. 4 year olds generally must be in a high back booster (but do not need the 5 point harness). I always check my full size boosters for my now 3 and 5 year olds. The airlines always allow them as additional baggage (in addition to my 2 bag per person allowance). You mentioned carry on bags and gate checking - don't know why you don't want to check in traditionally - but note that when United lost my sister's car seat (for a couple of days) they had a whole stock of seats at the airport and she chose one to use until they courriered hers directly to her.