If the word-final letter sequence <tion> is preceded by a vowel sound, the most common pronunciation of the sequence is ʃn. There are, as you would expect, exceptions. The word equation is pronounced ɪˈkweɪʒn by most GBE speakers. Some words ending <ition> are variable, some speakers using ɪʃn, and other using ɪʒn. The word transition is an example.
If the sound preceding the sequence is s, then the pronunciation is tʃn. as in combustion, digestion, question and the like. For other preceding consonants the pronunciation is ʃn, as in deception, invention, protection etc.
However, I have been noticing over the past few years younger speakers who use tʃn after consonants other than s, especially after n. So, for example, invention is pronounced ɪnˈventʃn by these speakers. I don’t know how widespread this is in the UK, or if it also occurs in accents other than UK English. I wonder if this is the beginning of a sound change which will result in a categorical difference between preceding consonant and preceding vowel environments.