You *should* be OK as the rules are on your side.
See
http://www.flyertalk.com/forum/briti...light-fco.html for some more info including links to 'rules' in post #28
I have had another reply from BA on this issue which is essentially 'yes you are entitled to access' but it hasn't really addressed the problem of lounge and customer service staff not knowing the rules (or the BA website or MMB being correct for that matter) or advice on what to do when denied. I'm hoping BA Executive CLub (Nicci) will be back soon and can revisit this issue.
Perhaps you could email Customer Services and ask and post back here? It would be interesting to know if their information has been updated....