All of the British armed forces would have contributed forces to Operation Downfall. Only the RN would make a serious contribution to Operation Olympic, the planned invasion of Kyushu; the RAF and Army would have joined in for Operation Coronet, the invasion of Honshu.

The Royal Navy's main force in the Far East was the British Pacific Fleet (BPF). Formed on the 22nd November 1944, it worked up with attacks on Japanese-held refineries on Sumatra. After this, it moved east, to join the US Navy. Operating from Manus Atoll, the BPF covered the western flank of the American invasion of Okinawa. Once this was complete, the BPF and USN made strikes on the Japanese mainland, with carrier aircraft and naval bombardments. In August 1945, it consisted of five fleet carriers, four light carriers, four battleships, eleven cruisers and 40 destroyers. There was also a large fleet train to support and resupply the carrier groups. Later reinforcements would have added the light carrier Ocean, specialised for operating night fighters. Two task groups would have been formed. One would have been based on the fleet carriers Formidable, Victorious, Indefatigable, and Implacable. The other would have had the fleet carrier Implacable and the light carriers. The British also had a fairly significant naval force in the Indian Ocean, Eastern Fleet. This was primarily concerned with carrying out Operation Zipper, the invasion of Singapore. Once Singapore fell, elements of Eastern Fleet would have been sent north to join in with Downfall. This would have included the battleships Nelson and the French Richelieu. The BPF included elements from the other Commonwealth navies. There were two New Zealand and two Canadian cruisers, as well as an Australian destroyer flotilla. By the time of Olympic, some 90% of the RN and its Commonwealth subsidiaries would have been in the Pacific to participate in the invasion.

The RAF's main contribution was to be a heavy bomber force, named 'Tiger Force'. This had originally been envisioned as a deployment of 20 squadrons of Lancaster/Lincoln heavy bombers. They were to be based at a newly built base in the Cagayan Valley on the Philippine island of Luzon. However, the engineering and shipping requirements needed for this base made it infeasible; it was also too far from Japan to be truely useful. The island of Miyako Jima (between Okinawa and Taiwan) was proposed as a base, but it had yet to be captured. The long campaign to capture Okinawa made an invasion of Miyako Jima impossible. Instead, space was found for the RAF on Okinawa. This meant Tiger Force had to be shrunk down to a force of 10 squadrons, but the decreased range to Japan meant that Mosquito light bombers could also be deployed. This would, ultimately, have been built up to a 20 squadron force over time. Tiger Force would have been composed of RAF and RCAF forces. The first engineering units to support Tiger Force were starting to arrive on Okinawa when the war ended. In addition to Tiger Force, there were also a number of RAAF and RNZAF squadrons in the Philippines and Indonesia, though they would not play a large part in the invasion.

The British and Commonwealth Armies would play the smallest role. Originally, MacArthur wanted the veteran 1st Australian Corps to participate in Coronet. However, the Australian government was facing political pressure at home to reduce Australian losses, meaning that the Corps had been committed to clearing bypassed Japanese garrisons in Indonesia, especially on Borneo. This problem was exacerbated by the draw-down of the Australian Army following the victory in Europe. These two factors cut the Australian commitment to Coronet down to a single reinforced division. Churchill, meanwhile, pushed heavily for a joint Commonwealth Corps of three-five divisions. This was to include the British 3rd Division, a Canadian division and an Australian division, with the latter two formed of volunteers. The composition of the other two divisions had not been determined at the end of the war. An Anglo-Indian division had been proposed, but had been rejected by MacArthur. New Zealand had been excluded from this original proposal, but was fighting hard to include their 2nd Division. The Australians were generally opposed to the formation of the Commonwealth Corps, which would likely put their forces under the command of a British general who had little experience fighting the Japanese. Had there been a delay with the British forces, they may have chosen to form their own Corps with the New Zealanders instead. It is likely that neither the Commonwealth Corps or this Australian Corps would have been in the first wave of Coronet. Instead, they would have formed the offshore 'floating reserve'. The Canadian Army was also preparing its own contribution to Coronet. The 6th Infantry Division was designated as the Canadian Army Pacific Force. Re-organized along American lines, and given American equipment (though retaining British-style uniforms), it had started training for the role in the autumn of 1944. The CAPF was originally intended to join one of the American corps, under an American commander. However, it may also have been integrated into the Commonwealth Corps.