This is quite an interesting one. Why? Because Expedia may well be correct in their calculations / terms & conditions, but I would be pretty sure that BA would be on the hook for a lot more. The reason being is that this area straddles your contract with Expedia and EC261. The latter suggests a full refund, no deductions. And here is the key: BA is responsible for EC261, not Expedia. However they will attempt to say "oh it's Expedia's problem, sort it out with them, we will give Expedia their money back". Which may be true, but the Regulation itself says it's BA problem, in words of (nearly) one syllable. So by all means exhaust the process with Expedia, and then pursue BA for anything you have lost. It may end up with CEDR . Expedia is likely to be the "fast" bit to this, in terms of expectations management.

There is one area which may be grey. I can't see why Expedia would do this, but it's possible that it's under 1 reservation but actually 2 bookings sown into one. If so - well best come back here or in the main EC261 thread to unravel further.

BA's website may well have limitations in these bookings, but ringing up will normally get you these sorts of combinations. Because amendments are free of fees in the first 24 hours, the easiest way to do this is to book something similar on BA.com then call up BA to amend it the way you want it, you would pay any fare difference, or get a part refund. Or a full refund if BA can't do it at all, which I doubt.