Unfortunately we are still awaiting news from Royal Mail as to exactly what will be available from the machines at the exhibition, other than the Sea Travel stamps.
My guess is that, with HM The Queen passing Queen Victoria's 'longest reign' milestone on 9 September, the additional inscription on either Machin or Flag (or both) will be
Long to Reign
More news here just as soon as we get it from Royal Mail.
UPDATE 31 August. I'm reminded that the Heraldic Lion stamp has been produced on a solo roll, to be used at the BPMA from September. It may be that this will also be available (with a special inscription) at Stampex:this early blog entry.