Well, let’s see:
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Belfast isn’t in England, it’s in (Northern) Ireland, and there are places in Ireland where you’d be made to feel very unwelcome indeed for daring to suggest otherwise.
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Batman and Robin were holidaying in Belfast when they realised they’d forgotten their capes, and obviously Bruce Wayne, being incredibly rich, keeps an airline on standby for just such an occasion.
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The boring answer - as I’m sure you know - is that Cape Air is one of the default liveries for the Britten-Norman Islander, and MSFS has neither Artificial nor any other kind of Intelligence for determining which liveries - or aircraft for that matter, other than perhaps not loading in airliners at grass strips - get shown at which airfields.