If they've reached a verdict on any charge and want the judge to know before they've finished with the others, they shouldn't communicate privately what the verdict actually is.
But this is a highly unusual jury, so who knows? Or we can say that at least one person sitting on it is a highly unusual juror. Whatever the verdicts are, and regardless of whether or not they reach a full set, this jury will become part of Old Bailey history, and English legal history, because of all the questions.
I don't know whether a majority direction has been issued, but assuming it has been and looking at this from a juror's point of view, possible reasons for telling the judge you've decided on one charge but not on the others would include a) wanting to be declared hung (on the others) so that you can get home ("we absolutely cannot agree"), and b) wanting to pressure fellow jurors. This is because the job is still to continue deliberating until you have decided on all charges.
PS I would have thought that until the foreman delivers the verdict in answer to the clerk's question it IS possible for a juror to change their vote. Not that I've ever seen this happen, or that there's any reason to think it might happen in this case.