In OTL before heading 14th Army on the Burma front, William Slim was in command of XV Corps after leading the bulk of the Burma army into India. he instituted new training for Eastern Army to reconquer Burma, then general irwin relagated him, disregarding his new methods and launched a traditional direct attack which failed. Irwin tried to blame Slim who had successfully got his troops out again, however irwin was sacked and Slim placed in command.
my POD is that Slim never loses command of XV Corps.
May 1942, Noel Irwin commander of Eastern Army comes down with maleria and is shipped back to Great Britain. Wavell impressed with the way Slim managed to extract most of his forces from certain capture by the Japanese and with the way he is improving both the capability and morale of Eastern Army promootes him to command of Eastern Army. Slim prepares to avenge Britain's earlier humiliation in the Far East. With the exception of Operation Ironclad in Madagascar Britain appears to be doing nothing to defeat Japan.
Resources are limited by the war in the Middle East and the need to escort Atlantic and Artic convoys so only a limited operation into the Arakan with the ultimate aim of Akyab island with its airfields- though there is severly limited naval resources.
Finally on December 21st 1942, Eastern Army launches the Arakan offensive. the offensive moves swiftly down the Mayu Peninsula with Slim having placed great emphasis on indirect approaches, resupply by air, concentration on attack and defence and complete integration between air and ground forces, they are also accompined by armour.
also to keep Japanese forces elsewhere in Burma distracted- a new group called the Chindits launches Operation Longcloth into Northern Burma, they are a special penetration group with missions to destrot for the time being Japanese railways. the Chinese also launch an offensive from Yunnan.
In Arakan British and Indian troops reach Donbiak where Japanese troops in concrete bunkers attempt to block their way. air power is used to help overpower the Japanese although many fight to the death. Commonwealth casulties are reduced through not making direct frontal assualts.
At this time, Japanese forces in Central Burma move down to defeat the the british in Arakan. However the British/Indian forces when surrounded form into protective boxes devised by Slim and are resupplied by air, with the allies having now reached parity and just slight superority over the Japanese thanks to Slim successfully appealing for some (though not many)
extra planes as the german threat to the Middle East diminshed. Slim also launched the rest of Eastern Army on an offensive from Assam into central/Northern Burma. Like the Japanese, the Eastern Army under Slims leadership crossed terrian which earlier allied commanders had declared was impassible leading to the earlier defeats. around the beginning of feburary the peninsula was taken and British troops landed on Akyab island with air support. The British Eastern Fleet aided this operation
although its resources were small, it shelled the island and lost one destroyer to Japanese planes.
with substantial parts of Burma retaken, plans were made to recapture the rest of the colony and hopefully beyond.
(So essentially the operation to retake Burma is begun sooner without the Arakan offensive being beaten back. I'm not sure how good this is, as info seems to be lacking but i'm going to try and finish this TL anyway, hopefully with better details.)