OP, on the Android-or-iPhone question: I'd stop treating it as the decision. Either one can be locked down properly for an 11-year-old, and the difference is only where the controls live. On an iPhone they're inside your own Settings app, under Screen Time > Family. On Android they're in the Family Link app on your phone, which is the bit others have described upthread (the bedtimes, the download approval). Pick whichever phone you're most comfortable being the parent of, because you'll be the one opening those screens every month.
What nobody writes down is the order, and it's the order that decides whether the bedtime actually holds.
iPhone: (1) set the Screen Time passcode yourself before the phone goes back to them, four digits they can't guess; Apple says it's a separate code from the one that unlocks the phone, so turn on Passcode Recovery in the same minute; (2) Content & Privacy Restrictions > iTunes & App Store Purchases > Deleting Apps > Don't Allow, so an app can't just be deleted and reinstalled; (3) test it on their phone, not yours: try to open something you've blocked. If it opens, the setting is on paper only.
Android: do every rule from your own phone in Family Link first, then turn off Developer Options and USB debugging on the child's phone, and check monthly that they're still off.
Two UK-specific things. Your network's 18+ content lock is a separate, free layer under the phone settings; on EE it's free and already on, and turning adult content back on needs a credit-card age check, so leave it alone. And Apple's own pages say age settings vary by country and one approval screen only exists on iOS 26, so write down the exact iOS version and the account country before following any guide, including this one.
Neither Screen Time nor Family Link reaches inside YouTube, Snapchat, Instagram or Roblox: each has its own controls and has to be done separately. And put a ten-minute recheck in the diary once a month; settings can quietly change after an update.
Full disclosure: I write parent guides for a living and one of them covers exactly this, but everything above is in Apple's, Google's and EE's own help pages, which is where I got it.