IMHO, the issue is that by attending the event, H's misrepresentation of his diplomatic importance was enabled by the UK, and the UK accrues negative publicity as a result and faces an additional, unnecessary, diplomatic minefield in an already volatile region.
H's agency could not enable him to attend a UK embassy event if the Head of State had advised all ambassadors worldwide that H is effectively persona-non-grata.
The UK ambassador should not have been presented a diplomatic quandary by his own Head of State due to H's status ambiguity. If H was PNG the minute Ted from WHO submitted his guest list for security, the issue would have been flagged, and a quiet no would have been communicated. And all embassies have security, especially in the Middle East. No embassy is hosting an event with surprise guests.
H can do all the media he wants. It is KC3's job to make sure his Foreign Office is kept out of H's hot mess.