Hmm, interesting deal, OP. And thanks for the follow-up tip on the London Travelcard.
Haven't looked at the fine print yet so I hope the offers will still be valid when we're in town in June. We've seen most of the key attractions on our list during a previous trip so we weren't going to pick up a London Pass nor were keen on paying a lot to repeat. But the 2 for 1 deals definitely makes it more palatable.