The hotel has sold out of its standard rooms and only has the upgraded room available for awards. So its a pain in that yes, as a Plat you could get a complementary upgrade to that very same room, but you first have to reserve a standard room to do that (and the hotel doesn't have any more of those at the moment.)

If you don't want to pay the extra cash per night, you might be better suited to look at one of the other Marriotts close by LHR.