You can apply for a place at Queen Mary University of London through Adjustment if you did better than expected in your exams, exceeding your current firm choice offer.
See the UCAS website for full information on how Adjustment at Queen Maty University of London works.
Once your original conditional offer becomes an unconditional offer in UCAS Track, you will see the option to register for Adjustment. You then have five days (including weekends) to secure a new place at Queen Mary University of London.