No its not only for the Clyde,its for any British or Irish port that the master holds the Pilotage licence for.
I have sailed on ships that displayed the said flag when entering or leaving ports where the Master held the licence.
It was most commonly flown on coasters trading around the UK.
Once again, John is spot on.
This extract [COLOR="Red"]here[/COLOR] shows the conditions for a Pilotage Exemption Certificate for Portsmouth and the link in paragraph 7(i) shows the flag (identical to the Polish national flag.)
Thanks for the explanation. I've seen the flag on several of the postings on here and always wondered what it meant, and only decided to ask now. Nice to know that even 'obscure' requests can be answered so quickly and accurately by you good people.