Is EC 261 the right angle to take here, or is it better dealt was as Consumer Right Act 2015 matter? Ultimately, you had a contractual relationship for a service that you did not get. Taking this angle might mean BA doesn't have the stock defence ready to go, and would therefore need to spend time and money preparing for it. This might convince them to settle (or settle on better terms).
If you do persist down the EC261 route, the T&Cs say this:
Originally Posted by Terms and Conditions
- The Companion Voucher allows the main British Airways American Express Card Account holder, when making a flight booking using Avios, to book another seat on the journey for a Companion without having to pay the Avios flight price for that Companion.
So perhaps there are no miles attributable to the other passenger's costs. But what happens with the voucher? Surely that's fully attributable to the other passenger. EC 261 says the reimbursement is 75% of the "price of the ticket". Does that include 37.5% of the monetary equivalent of the voucher?
I'm not a lawyer - but you might have legal insurance as part of your travel insurance?