Do you have Amex? There is a service called priority lounge or something like that and it gives you access to many lounges worldwide even if you aren't on a particular airline. Some airlines allow day passes, such as United, if you are flying on them. Another way to get in lounges is to get a credit card that allows club access. United has that as well. I dont believe you need to travel on them. I am not sure if BA has day passes.
