Again, you're getting it wrong.
ADP is an optional extra layer of protection that applies E2EE on extra categories of data, meaning only the owner of that data (the user) can access it via trusted devices. Apple cannot access it, even if they were served with a valid warrant.
SDP (Standard Data Protection) covers other categories, some are covered by E2EE, and some are not. Those that are not can be decrypted by Apple, and therefore are available to law enforcement agencies if Apple is served with a valid warrant.
It's all explained here:
https://support.apple.com/en-us/102651
Right, now that's out of the way... Yes, removing ADP means Apple can decrypt data
if required to do so via a warrant.
The thing you're getting wrong is that the UK government requested that Apple add a backdoor to their E2EE. If Apple had done this, the UK government would be able to decrypt any iCloud data for any user anywhere in the world. It wouldn't have mattered if the US government had asked for the same because a backdoor is never going to stay private and secret to a government. Hackers will find it and exploit it. Governments in countries you don't like, say China, would be able to access the data.
Adding a backdoor means effectively removing encryption.
Apple didn't do that, and they won't do that.
Right now, the UK government has to continue requesting specific data form Apple via a warrant. That's it. If Apple had added a backdoor they wouldn't have needed to bother. Imagine a UK law enforcement officer just randomly wandering through your data, finding something they think is illegal, then charging you with a crime.
Had Apple agreed to the backdoor it would set a precedent, and governments would start going after other E2EE systems, such as WhatsApp. Yeah, no more E2EE chats.