Hi mscarton,
welcome to te Apople (user-to-user) Community!
Re: "The slider to turn off Apple Intelligence is not shown on my iPhone (15 Pro Max) when language is set to English(US). If I set the language to English(UK), it shows up and I can disable it."
Am no expert in this area. yet your request deserves some response. Your Company's.policy needs compliance.
Wondering if this is where you have been seeing that "slider"
Apple support's online article: Get started with Apple Intelligence on iPhone - Apple Support
Eg: says:
"Turn Apple Intelligence off:
Go to Settings > Apple Intelligence & Siri, then tap the button next to Apple "Intelligence."
(Note: nothing there about language settings being necessary, or prohibitive to turning off?)
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Re: "So if I switch to UK, disable Apple Intelligence, then switch language back to US, how can I verify that Apple Intelligence is still disabled?"
The above linked article has a list of features under :"Use the new Apple Intelligence features"
These features may only be usable when Apple Intelligence is not turned off. That may be one way to check.
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Re: iOS 18.3
Seems Apple Intelligence is on by default after downloading iOS 18.3 , unless it was turned off previously.
Turning Apple Intelligence off may delete some of the 7 GB (or so) software downloaded for use. I don't know for sure if or when that applies ... you could see if the amount of " free storage space " increases by some GB when you turn Apple Intelligence off. Seems that may indicate the disappearance of Apple Intelligence software.)
Re: Screen Time Settings: If useful to you (maybe in the short term):
Seems Apple Intelligence features can be blocked using Screen Time settings:
Block access to Apple Intelligence features in Screen Time on iPhone - Apple Support
An overview of the latest iOS 18.3 Apple Intelligence features: iOS 18 - Apple
If needed: There are links to Apple Support at top right, and at bottom left, on this discussion page
or
See a list of Apple phone numbers around the world.
Contact Apple for support and service - Apple Support
All the best :-)