As Kjn2 indicates, BA's pricing is even odder than that of the 'low-cost' carriers and it's hard to predict which way their fares will go.
As you're obviously wanting to fly from London, here's a a list of all your options:
BA from Heathrow.
easyJet from Gatwick or Luton (which replaces Stansted as from next month)
Norwegian Air Shuttle from Gatwick
Scandinavian Airlines from Heathrow.
Remember though that Ryanair fly to Malmo, which is just a short hop over the Oresund bridge from Copenhagen. You can either get the direct bus to Copenhagen from Malmo airport or take a bus into central Malmo (which is a lovely city to explore) and then get the train over the bridge. (They run every 20 minutes because so many people live in one city but work or shop in the other). I've been to Copenhagen that way (using the train outbound and the direct bus to Malmo airport on the way back) and I can assure you that it's very easy to do.