The reason for that is very clear. Specifically in case of Iran, I must remind you that Britain caused the loss of so much of Iranian territory. Iranians have not forgotten that. Then British with the threat of war got Iran to surrender its hydrocarbon resources in 1901 to a British company which later became BP. This British company used to pay Iran only 16000 (Sixteen Thousand) sterling pounds yearly for all the oil they could remove from Iran, which was the only oil resource for British empire and UK used it run its colonies around the world as well as switching its economy from coal to oil. Not to mention the same oil was used by UK to fight two world wars and creating a world in which British dominance was ensured. All the while British living standard was improving because of this oil while Iranians were living in abject poverty.
When finally Iranian rose up and elected a government to get back their oil after 52 years of exploitations, British colluded with Americans to take over Iranian oil again. After operation Ajax, again Iranian oil was divided among powers with US taking 50%, British 30%, France 10% and Iran only 10%. The stealing of oil never stopped but Iran's share dramatically improved from 16000 sterling pounds a year to 10% of oil revenues, still peanuts though. This continued till 1979. The strange thing is UK only started to exploit its own oil resources once Iranian revolution happened with UK drilling Brent in 1980. Then to take revenge on Iranians they started supporting MEK, Saddam, and God knows who. What a shame and thief nation, this is. It is entirely made up of stolen goods from all over the world. Ofcourse Iranians are angry and will remain until they can get all that oil back with interest, which I am sure is going to be thousands of trillions of dollars.
This is just one part of the story. Part of the story of Iran. And I am not going to start what you stole from Indian subcontinent. I am not talking only about Kohi-Noor diamond. It is more that. You see your nation engineered many famines in subcontinent, which killed tens of millions of people. For example the famous Bengal famines of 1943, when British troops destroyed crops and set alight grains and food to kill as many as local population as possible just like famine of 1771. People have not forgotten such things, just like Iranians have not forgotten that British troops in second world war used Bistoun mountain writings in Iran, a unique historical site for shooting practices, destroying the ancient writings on them. No, dear people have not forgotten, and if a another nation had done all these to you, neither would you.