Some reporter asked Matt Hancock how long was a reasonable length of exercise, and he suggested half an hour to an hour for most people - that's probably where it came from. It's not official guidance and certainly not a hard and fast rule anyway. From what I recall the once a day part is also only a guideline and not actually the law in most of the UK (except Wales).
https://www.gov.uk/government/publications/full-guidance-on-...
"one form of exercise a day"