Lonnie Donegan Well well!
Working in the family music shop our manager (or my grandpa depending who got there first) used to alert us behind the counter whenever anyone important walked in, and just before I served him (probably with guitar strings, which was our normal sale generally) the manager rushed behind the counter to tell me the man I was about to serve was Lonnie Donegan.
I could write a long list but will only add one I picked up on my own, Jeremy Taylor who switched between South Africa and Britain (he was on That's Life in the 70s), who got the Number one Ag Pleez Deddy in 1961 and I told him how much I liked it. He was a friend of my grandpa's (they all worked together at some point, he was a guitarist for over 70 years), and have exchanged music by post with him since.