The map of southern Thailand, especially the Pattani region
(provinces of Pattani, Yala and Narathiwat)
The conflict in Southern Thailand escalated in 2004 (though the violence persisted on the earlier decades). One of the root cause of the insurgency is the 1909 Treaty between Thailand and Great Britain.
The Anglo-Siamese Treaty of 1909 was signed on the 10th March of that year in Bangkok between the British government and Siamese (now Thai) government. The ratifications of the treaty were exchanged in London on 9th of July.
The agreement effectively divided the northern Malay states into two regions; the Pattani region (now provinces of Pattani, Yala ang Narathiwat), as well as Satun and southernmost part of Songhkla province remained under Thai control, while Bangkok relinquished its claims over Kedah, Kelantan, Perlis and Terengganu. The only defect is the Malay people were not represented in that treaty.
Because of the treaty, it defined the Thai-Malaysian border.
So, what if the Pattani region (historically the Sultanate of Pattani) were given to British Malaya instead of remaining Thai? Will it affect the relationship between Thailand and Malaysia? Will the conflict in the Pattani region never happen like in OTL? And additionaly, if Satun became part of British Malaya, will it merge with Perlis?