World War II was not kind to Great Britain. With its Empire already unraveling and its economy still recovering from the Great Depression, it went into WWII and suffered greatly. Though it won the Battle of Britain, the collapse of the Soviet Union left Britain isolated. The Anglo-American bomber offensive was defeated in Mid-1944 by massed German Jets. The introduction of the type XXI Elektroboote Stealth Submarine fitted with guided torpedoes won the Battle of the Atlantic, and Britain was starved into surrender, reports of cannibalism being common during the last months of the war.
Under the Terms of the Treaty of Stockholm, Britain was forcibly incorporated into the National Socialist Federation, and lost Northern Ireland to EIRE. Though it managed to gain a great degree of autonomy after the death of Hitler, it is still a military member of the NSF, and the Royal Navy is viewed as a serious rival to the US Navy.
In recent times, there have been secret meetings between members of MI6 and the OSS, and Britain now has a far more favorable outlook of the United States. How Nazi Germany will react to this remains to be seen.