Whilst I agree with the other responses that you may not necessarily been mis-sold, I think there is a clause in the Consumer Rights Act whereby any information provided to a customer, whether verbal or non verbal is considered to be a term of a contract if this is taken into account when making a decision to purchase that service/contract.
I'm not a lawyer, but given you consulted with multiple staff members and BA have admitted that they made an error it may be worth looking into if you don't have other options to reach silver before your year ends.