I’ve had this happen with a retailer that sells codes for things like gamepass, Xbox live, iTunes vouchers etc.
The issue was I purchased an abnormal amount. 4 3 month gamepass cards to total 12 months. They wanted me to send copies of my passport via email, so I noped out.
Never happened with John Lewis, but my guess is it’s something similar and was prompted by the quantity of iPads you ordered, which warranted stricter checks. Retailers do this if they suspect something like fraud or scalping.
I could be mistaken here, but credit reference agencies have been able to use your name, address, and debit card number (or just the digits unique to your bank), to match you to your credit file. I’m guessing John Lewis tried to do this, but due to Apple Pay were unable to, so couldn’t complete an AML check.