It would be as wrong not to assign a high grade to a student whose documented work supports it (regardless of what they need to achieve to make their firm offer), as it would be to assign grades not thus supported, to a student who was predicted aspiratioally to make sure that they got an offer for a course / university that they wanted.
e.g. Student A ranks on all parameters below Student B, but Student A has to have an A to make her offer and Student B doesn't; it would look suspicious if we predicted more than x As, so we'll bump Student A up to the A* group, but shift Student B down to the As.
In other words, it's just plain wrong not to be honest at this stage. As (honest, not wrong, as) the grade achieved in the actual A level exams would have been.
And @goodbyestranger, no, of course they can't - but a student who was at A for most of year 12, say, but predicted an aspirational A, could perfectly reasonably up their game and demonstrate the likelihood of getting to the A, in the 7-odd months between prediction and exam. In this case, it would be wrong not to give the A* because of a blanket policy not to give the 'aspirational' grade predicted for UCAS. If nothing else, that policy shouts out that the school has been a little dishonest at the grade prediction stage, I'd have thought?