I can address North American phonology in detail, but not British phonology.
[j] is not standard in the pronunciation of the word immediate.
Source: New Oxford American Dictionary - immediate [ ɪˈmidiət ]
Source: Oxford Learner's Dictionaries web site - immediate [ɪˈmiːdiət] (AmEng)
If a speaker were to insert [j], it would normally assimilate with the preceding [d] and become [dʒ]. I have heard regional pronunciations of "immejit" [ɪˈmidʒət].
The schwa is a (variable) central vowel. It varies (assimilates) depending on the phonetic environment. Here [ə] becomes [ɪ]. So, immediate [ ɪˈmidiət ] > [ ɪˈmidiɪt ]. A [j] doesn't normally occur as the tongue transitions from [i] to [ɪ] to [t].
Another example of schwa becoming [ɪ].
Oxford Learner's Dictionaries web site - wanted [ˈwɑːntɪd] (AmEng)
Given the [t] before the schwa and the [d] after the schwa, the schwa becomes [ɪ].
Here is an example of [i] becoming [j].
Source: New Oxford American Dictionary - California [ˌkæləˈfɔrnje]
In brief, the final vowel is a schwa, and the sequence [niə] reduces to [njə].
Curiously, the (British) Oxford Learner's Dictionaries site gives California [ˌkæləˈfɔːrniə] (no [j]) but has an audio recording of a native speaker with an easy-to-hear [j] (which is correct, I am from California).