Hello, thanks for your message I appreciate it is important to you. I have taken a look at the above correspondence plus your conversation with Jim, and must say I am in agreement with his opinion on the matter.
In essence, when you book a flight through a platform such as Expedia, whether you have a cause of action against them will be determined on how the booking was made. Expedia is a platform which does advertise flights from different airlines, however I note when booking through their website, some flights will take you directly to the airlines website to complete the booking, whilst other flights you can book directly from Expedia's website.
It is unclear from the information where the booking was actually made i.e. was it on BA's website directly or was it on Expedia's website. If the booking was made on Expedia's website then you would have a cause of action against them as agents on behalf of BA, but if it was made on BA's website, then you would have a cause of action against BA only and Expedia would hold no liability.
However, discussing a cause of action, in essence the only viable one here is one in contract and from what I have read from your previous information there has been no breach of contract by either party, please correct me if I am incorrect. The issue effectively is you cancelled the flight due to the pandemic and now require a refund, and you are not requesting a refund due to either party breaching a contract against you. The request for a refund therefore would not arise from a breach of contract, and if you are successful in court it will be for the court to decide whether you are entitled to a refund due to a frustration of the contract, as opposed to a breach.
However, in my opinion it is premature for you to file a notice of discontinuance against Expedia for now until it has been established that they had no role in this matter, this information is still unclear to me as normally any company acting in the capacity as agents, such as Expedia would hold a contractual relationship with the customer, but as mentioned it just depends on where the booking and payment was actually made. If it was made on BA website and you visited this via Expedia then BA would be liable for any refunds. But if the booking was processed directly through Expedia then they would hold some liability as agents on behalf of BA.
I hope this goes some way to assisting you and wish you well.