It seems that on 21 June 1948 – the Small Scale Experimental Machine (SSEM), which was nicknamed the Manchester Baby – executed its first code.
The system was designed and built at the University of Manchester by Frederic C. Williams, Tom Kilburn and Geoff Tootill.
Apparently it used a 32-bit word length and had a memory of 32 words (1 kilobit), and was originally intended to be a test for a form of electronic memory known as the Williams-Kilburn tube.
Its significance is such that The Baby can fairly be described as the antecedent of all modern computers, be they desktops laptops or iPhones. So all honour to Manchester! And Happy Birthday Baby! So it’s a landmark for the world, not just the UK.
“The Manchester Baby was very limited in what it could do, but it was the first ever real-life demonstration of electronic stored-program computing, the fast and flexible approach used in nearly all computers today,” said Dr James Sumner, lecturer at The University of Manchester in the History of Technology
What was the first program? To find the highest proper divisor of 2 to the power of 18 (262,144). It took 52 minutes to run through 3.5 million calculations, but it got to the correct answer.
See more on the University of Manchester website, in an article from yesterday, How a 70-year-old ‘Baby’ changed the face of modern computing, and 2008, Birth of first modern computer celebrated in Manchester.
You can read more about the SSEM on Wikipedia, too.