"Well, compared to Manchester" = "at least it seems that way when you compare it to Manchester" This is not what people refer to when they talk about "passive voice" because "comparison" is not part of the verbal action in the sentence. It is not part of the "voice" of the sentence, so it can't really be "passive voice". That said, there is still something passive about it. Whenever you use a past participle as an adjective, you always create an unseen actor. In this case, the unseen actor would be the person or people who compare Worcester to Manchester. Who are they? It really doesn't matter, but since there is an unseen actor there is something passive about it.

It’s not the past tense but more of a passive/ adjectival tense

Note that the more interesting question here is the preposition after ‘compared’, not its form. You can look up the subtle differences between ‘to’ and ‘with’. In speech, both would sound normal here.

They might have written "in comparison with Manchester" or even "so different from Manchester". Neither of these expressions have anything verb-like about them, which may help explain why "compared to Manchester" appears verb-like, but isn't.

This is very common, but I had to look up what it's called. I believe PAST PARTICIPLE ADJECTIVE is the term. Almost any verb can be used as an adjected this way: He lives in a painted house. We ate boiled cabbage. The example above uses this, it's just a little more complex, since the 'adjective' is a phrase e.g. 'compared to . . . " Preset tense does not function as an adjective.