Of course you will have lounge access.
It's an MFU but that doesn't mean that you didn't pay for your miles-you got them because you paid BA money for flights and enough of them entitle you to upgrade to whatever class of travel you have enough for subject to availability.
Your ticket when you do that is no different to an all money ticket in terms of the service you should get including lounge access (except as said,the miles and tp's you earn are based only on the type of ticket you paid money for).