What does “over-promoted bath toy” in Hugh Grant’s tweet exactly mean?

Hugh Grant, British actor called the U.K. Prime Minister, Boris Johnson, an ‘over-promoted rubber bath toy’ in his tweet on August 29, which reads:

“You will not fuck with my children’s future. You will not destroy the freedom my grandfather fought two world wars to defend. Fuck off you over-promoted rubber bath toy. Britain is revolted by you and you little gang of masturbatory prefects.”
Source

I first thought the phrase, ‘over-promoted rubber bath toy’ meant immediate Brexit with no conditions attached. But the next day’s ‘insider.com’ annotated the twitter as:

The actor called the PM an over-promoted rubber bath toy and his cabinet a little gang of masturbatory prefects.

What does ‘over-promoted rubber bath toy’ exactly mean in Hugh Grant’s context? What does “rubber bath toy” have to do with the new British Prime Minister?

Does ‘over-promoted’ mean “overly advertised / publicized,” or does it mean his “premature ascending" to high position as Prime Minister of a great country?

I think you need to allow for the possibility that "over-promoted" and "rubber bath toy" are not necessarily conjoined.

It seems to me that Grant's use of "over-promoted" is a commentary on the job Johnson now holds—he's in over his head as PM, has been advanced beyond his depth. The "rubber bath toy" is just a simple insult mocking Johnson's puffy appearance and wacko hair.

This makes more sense than trying to apply "over-promoted" to the toy.

I don't think that in this case over-promoted is used in the metaphorical sense of being hyped too far. I think it means, simply, elevated in rank above the individual's capabilities. History books seem to be littered with soldiers over-promoted to general rank.

For a rubber bath-toy to be elevated to Prime Minister of a great country would prima facie be a case of over-promotion.