Who managed the longest period between their first ever release & their first number 1 single?
Looks like, in the US it was Santana with 'smooth':
The record for the longest wait from an artist's Hot 100 debut entry to its first #1 belongs to Santana, with 30 years between the time he first cracked the Hot 100 with "Jingo" (October 25, 1969) and the first of 12 weeks at #1 with "Smooth," featuring Rob Thomas (October 23, 1999).
In the UK I couldn't find that stats for that but the longest between number 1's is The Hollies at 23 years:
The Hollies went 23 years between their first top hit "I'm Alive" in 1965 and their next top hit, the re-release of "He Ain't Heavy, He's My Brother" in 1988. They had eleven other singles in the top 10 in that interim time, including three singles to reach the number 2 spot.
A bit outside the requirements (single -> #1), but you can maybe consider anyway :
"Sugar Man" by Sixto Rodriguez
This song wasn't released as a single but was part of an album in 1970... and didn't really reach number 1 in 2012, but I think almost everyone heard it then on the radio/tv!
You can see why by watching the movie Searching for Sugar Man (I can't sum it up... it would spoil the movie & that fascinating story)