Not a long wait....unless there's a subtle difference between "check-in" and "seat choice"; if there is I'm wrong, but I don't believe that you should need a law degree to interpret the (deliberate?) ambiguities of a website purchase process.
By the way, I would like to meet the management cretin who thinks that "availability of food and drink to buy", as opposed to "free refreshments will be offered during the flight", or "there will be no food and drink on board, free or for purchase", is a huge and sought-after benefit, even if it is M&S. (And even that shows a peculiar mind-set; what's so wonderful about M&S, as opposed to, say, Sainsbury's, or Fortnum & Mason's, or even Tesco's? M&S is good for underwear if you don't care what it looks like and are in a hurry, but no better than anyone else in other departments. Why would a non-Brit be encouraged to fly BA just because customers are given the opportunity to pay a lot for M&S food on board? Or a Brit, for that matter?)