This is an easy credit card dispute. It's not for you to worry about who holds the funds as between Expedia, BA and EI. The three of them can work it out as they do every day.

Under EC 261/2004 and the COC you were entitled to cancel the ticket for a full refund because it was cancelled. As the operating carrier of the cancelled flight, your claim, if you wished to pursue it, would be to EI and it would be for EI to track down BA or Expedia and make certain that the refund is made or to make the refund and then deal with Expedia/BA itself. Again, not your problem.

But, why bother making claims which will take forever when the CC dispute is easy and fast. Whoever the merchant vendor was will receive the chargeback notification from your card issuer and it will be for that vendor to figure out where the funds have gone, but it will be out the funds in the meantime.

As a rule of thumb, make the request, give it 7 business days and then initiate a charge back on the 8th day, noting that your request to the carrier has not been satisfied.

All of this aside, yet another reason not to deal with third-parties such as Expedia. In the end, unless you work at poverty level wages, this ticket will not have been cheaper, it will simply have cost less direct funds.