For a perhaps limited time, under 16s can enjoy a first class experience in the London area for well under £1, on one of the nicest 1st class environments in the country. I know of two forum members who regularly take advantage of this.
As for what the staff will think of a teenager asking for a snack box for a journey between Waverley and Haymarket, I really wouldn't like to predict the outcome!
A while back on a VT loco hauled set i was travelling Glasgow to Motherwell on a standard open priv and asked for free breakfast ! They were having none of it . Changed at Mwell onto GNER to Edinburgh . Joys of £2 priv and first class at weekend .