I now recall buying petrol at a Shell station just off the M6 last year, so I've answered my own question about whether I converted Shell points to Avios in the past 36 months. So as ankomonkey said, it looks like a targetted email. I assume that BA has sent it to members who converted from Shell to Avios in the past 36 months, which would explain why Shell Drivers Club isn't mentioned in the text of the email, because they already know that I did!