Roland,
How are the Iranians going to determine which tankers are carrying oil for the UK, as opposed to France, Germany, etc, etc....
Close the Straits of Hormuz and you have the entire global community down on your neck....not just the 9 MRA4s of the RAF!!!
Regarding N Korea, while I don't like to quote wikipedia as a totally reliable source, I believe the figure of 80 P-3C for the JMSDF is pretty accurate...
Japan Maritime Self-Defense Force - Wikipedia, the free encyclopedia
As for China, their livelihood is becoming increasingly dependent on us (the western world in general) buying the consumer goods they are producing in massive quantities, so why do they want to go to war with us......????
For what it is worth, I believe the UK should have an MPA fleet, but not for any of the reasons you have quoted!!!