There is no way that the average poorly COVID patient, at the height of the pandemic, would be in an ITU bed with one to one care from qualified and very expensive staff if they didn't require ventilatory support, and we know that Johnson was never ventilated.
Now, there's an argument that, for the greater good, the leader of a country, in a time of extraordinary crisis, should have medical and nursing care over and above what is strictly clinically required (and tbh that happens not infrequently with VVIPs in my experience).
It's also quite reasonable that a very poorly Prime Minister should be cared for in a separate room with dedicated staff for both security reasons and to avoid disruption to other patients/staff. We know Johnson was in a separate room adjacent to the ITU.
It seems to me most likely that, for expediency and because this was a VVV VIP, Johnson received ITU care, although not fulfilling the clinical criteria for such care.