If you do not have ADP enabled (which is the case in the UK as of now), device (iPhone) backups are not end to end encrypted and are stored on Apple's systems unencrypted (or encrypted with a key that Apple knows).
If you have ADP enabled then device backups are end to end encrypted; only you have the keys and therefore only you can decrypt the backup.
If you do not have ADP enabled (which is the case in the UK as of now), device (iPhone) backups are not end to end encrypted and are stored on Apple's systems unencrypted (or encrypted with a key that Apple knows).
If you have ADP enabled then device backups are end to end encrypted; only you have the keys and therefore only you can decrypt the backup.