e3fc0434_anced_seat_selection___Gadling__Premium_Services
[0] British Airways [Earliest available] Free at check in or from 24 hours before departure [Charge?] Depends on class of service; For international economy and domestic UK, £10/$15 to chose seats from time of booking up to check-in, 24 hours before departure up to £60/$90 for other classes of service (First Class free) [Can you pre book an exit row?] £50/$75 for economy/World Traveller Plus [Premium Services] 
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Once a free perk, many airlines now charge for advanced seat selection - Gadling
Premium Services