In 1940, all hope was lost for Britain, her dominions were gaining independence, Nazi Germany was on the brink of invading her lands and her economy was in huge trouble, then one lucky day, an alien spacecraft, unknown to man crash landed near Portsmouth harbour. The authorities were immediately informed and the local police came to investigate. Inside the ship was a transport containing 10,000 Alien creatures, sent into exile by their tyrannical masters. Soon the Army arrived and transported the creatures to a hospital to be treated. Within days, the incident grabbed world wide attention and scientists recovered technology from the craft.
The idea of allowing British Citizenship for the alien arrivals spurred throughout the government, Winston Churchill then announced that the Aliens can claim to be Asylums in the country and made a personal visit to the crash site. Many religious minorities opposed this, yet were ignored. Hypersonic Jet flight, laser weapons and interstellar space travel technologies were reversed engineered for the war effort, national debt were wiped of the face of the Earth for the Britons as the economy skyrocketed relentlessly.
World War 2
Just about a week after the crash, a fleet of hypersonic laser-armed spitfires were deployed across the English Channel, entire Luftwaffe squadrons were wiped out in a instant and Laser armed tanks and other alien technologies were already being deployed in North Africa. North Africa was liberated in five days and a much earlier and easier d day was commencing in Normandy, the moment the British arrived at Sicily, the stunned Italians surrendered and the country was occupied too fast for the Germans to grab hold of.
Paris was liberated just days after D Day and Berlin fell easily two weeks later, ending the war in the west.
After the war
Britain had primarily given back the French colonies just after Germany surrendered, France however thought this wasn't enough and demanded some technologies to be supplied to France. Winston Churchill refused any demand given by any country and even annexed all dominions by force to show off Britain's power and willingness to fight. Britain began invading Japanese possessions in the Pacific and annexed them without giving them to their rightful owners, even the Philippines which belonged to the USA. Britain then turned its attention to the rich oil lands of the middle east, each country one by one fell in an instant.
After the war, the British Space Agency was founded out of huge funds by the British government and reversed engineered the engine of the Alien craft that crashed years earlier. Eventually in January 1949, the first human being went to space and took the first pictures of Earth from space. By September, the first man on the moon planted the Union Jack on the lunar surface and set up a colony there.
In 1950, Britain's space colonisation efforts continued with the launch of "Victory" mankind's first satellite, and the construction of Churchill space station. Back on Earth saw the foundation of a democratic Germany with 1937 borders, this new Germany however were Pro-British instead of Pro-American as in OTL.
The Alien population in Britain from the crashed spacecraft were offered a job at the British Space Agency to which they accept, by this time, they have already learned English and English culture.
The British government declared war on the Chinese Communists in the civil war and with their technology, victory was inevitable. Other nations especially the USA expressed jealousy for the British, believing that they should be given the glory.
In 1952, Winston Churchill declared that a manned mission to Mars shall be assembled with the goal of exploring and colonizing the Red planet with the Alien technologies. On July 25th, the Union Jack was planted on the Martian soil and a colony near the Martian Arctic was established which over time grew into a city by 1955.
By 1960, London has a HUGE skyline and technology is 50 years ahead with even further advancements in space travel, Churchill Station is now a space habitat with 10,000 inhabitants, Britain remains the sole superpower of the world and hangs on to all her colonies. The Britons have already set up outposts on the far reaches of the solar system. The British Empire includes the Middle East, Canada, Australia and most islands in the Pacific.