Hi and thanks to all who helped on my previous queries!

I am flying LHR-SFO on Friday, but have a BMI ticket (it was much cheaper!) and have been ticketed J. Can I use miles to UG to F? If so, should I do this now, or try and do at departure? I'm not given the option on .bomb, but I assume this is because it's a BMI ticket.

Thanks again for the help!