A lot of Britons are hugely fond of Thailand, but relations between the countries have had difficult times as well as good times.
As early as 1826, when we had just finished fighting the Burmese for the first time, we signed a Treaty of Amity and Commerce with the Kingdom of Siam. And in 1855 we signed a Treaty of Friendship and Commerce between the British Empire and the Kingdom of Siam. It all sounds terribly friendly and amicable (with quite a lot of commerce included as well, presumably), except that there was already a sense that growing British power in the region and British desire for commercial and political advantage was putting pressure on Thailand. Indeed, the Anglo-Siamese Treaty of 1909 transferred territory from Siamese to British control.
Nevertheless, Siam joined our side in the First World War in 1917.
In the Second World War, Thailand came under pressure from the Japanese and declared war on Britain. However, there were many Thais who resisted the Japanese presence in Thailand. Then in 1945, with the Japanese retreating, we were planning to invade Thailand, and British and Indian forces mounted attacks into Thailand (like a raid on Phuket in 1945) and advanced across the border in the summer of 1945, but as the war came to an end we abandoned our grandiose schemes for invasion.
In the period after the end of the war, British and Commonwealth troops were deployed in Thailand to help disarm Japanese troops and to assist liberated Allied prisoners of war.