I'm not sure why you'd assume this to be true; Britain faced several terrorist movements in India throughout her occupation there - I mean from early revolts like the 1857 Sepoy uprising all the way up to the violence which gripped the country on the eve of partition. The most obvious later example would be the attack on Chauri Chaura police station in which some 22 policemen were burned alive. You even see some very isolated examples of terrorism within the home country that looks a little like what we'd expect today. In 1909 Madan Lal Dhingra murdered Curzon Wyllie, an Indian army officer, in London! Disaffected Indians even sided with the Germans (bose, for example) and the Japanese during the war (though, to the credit of the Indian people, the vast majority of Indians came to Britains aid in her time of need!) A lot of violence was averted in India thanks to the widespread politicisation of non violence, exemplified most famously by Gandhi who repeatedly condemned acts of terrorism when they occured. This is all not to mention the horrific bloodshed and violent terrorist attacks perpetrated by Indians against their religious enemies which continues well into the current century.

As for why it seems different to modern terrorism, you have to remember that this is an entirely different time. If you were an Indian nationalist considering an attack against Britain you'd have to pack all your (likely poorly made and volatile) explosives and weapons onto a boat and sail for several days to reach Britain, all while keeping your motives concealed and your weapons in good condition and without any form of contact with any terrorist cells in Britain or India. This is far, far harder than simply contacting a local cell via the phone or internet and organising for someone to drop a backpack full of fertilizer outside an embassy or in a metro station, and even then attacks like that today are vanishingly rare!

There were, especially with the Spanish, but also with the US and everyone else. Spanish-Chamorro War on Guam and the other Mariana Islands (not really in SE Asia, but close, and they are the same ethnicity/ culture) resulted in 90% of the local population dying of disease and in conflict. Also the Philippine-American war was a pretty big one, and led to disease and massacres resulting in nearly 1/4 million Filipino deaths.

"I want no prisoners. I wish you to kill and burn, the more you kill and burn the better it will please me. I want all persons killed who are capable of bearing arms in actual hostilities against the United States.” General Jacob H. Smith said.

So in many cases it was either so early in history that no one cared, like with Native Americans, or its simply not talked about in the West. They were still dying of Western diseases during this period as well, showing how one ago this was. Also these places were separated by water and language, they were often dealing with different occupying forces, and they were not being armed by other western countries. If it was like that, we would probably have seen something more similar to the Vietnam War, I believe.

"You never hear of any disturbances in Northern Luzon; and the secret of its pacification is, in my opinion, the secret of the pacification of the archipelago. They never rebel in Northern Luzon because there isn't anybody there to rebel. The country was marched over and cleaned out in a most resolute manner. The good Lord in heaven only knows the number of Filipinos that were put under ground. Our soldiers took no prisoners, they kept no records; they simply swept the country, and, wherever or whenever they could get hold of a Filipino, they killed him. The women and children were spared, and may now be noticed in disproportionate numbers in that part of the island."--From a Republican Congressman, who visited the Philippines during the summer of 1901 Boston Transcript, March 4, 1902[19]