Thanks for the answers, they help narrow down the options. As mentioned all you can do in this case is call Ba and ask them if anything can be done explaining that you went to the airport and checked in but then felt unwell and told the agent that you did not feel you could travel.

There is a generic clause in ba’s coc about being unable to travel for reasons outside of your control and telling Ba as soon as possible. I’d argue you did the second part though you don’t really have any proof that you were unable to travel in the absence of medical diagnosis so that weakens your case if they want to be tough. Hopefully though if you speak to an agent which takes your word for the inability to travel I hope they will help somehow. If the ticket was cancelled as no show, it may not be possible to reinstate it but maybe there is a way of giving you a refund and you’d have to purchase a new (and possibly more expensive) ticket. You can only try anyway.