Surely going with a family is much easier since every family member has their own baggage allowance..

Not to mention that in my experience families wear the same clothes all day right up until their 9.30pm bedtime and are quite often even seen in the same clothes on two consecutive days. Whereas I need 3-4 complete changes of clothes per day. (Thinking of last weekend, typical would be lunch-wear, going-to-meet-friends-at-their-pool-wear, shopping-wear, dinner-wear, club-wear; or on Sunday brunch-wear, club-wear, another-club-wear).

Slightly more on-topic: one good thing about the BA bag allowance is that although there are volume restrictions there is no weight restriction unlike on many airlines (although you are supposed to be able to lift in into the overhead lockers). My hand luggage normally weights over 10kg, which effectively gives me a 33kg allowance in total on a European weekend trip in Y, which I find is enough (just) even for 3.5 days.