There's more to it than just the flag issues. 231/233 are fully equipped for Northern operation (bar MMI socket and screen, which is an oversight but there must be a reason somewhere). Thus they are more suited for Enterprise duties. The others (in this case, 215,217-226,229,232,234) do not have NIR safety equipment (such as AWS, TPWS, NIR Train Radio and isolation bars) and would require a secondman. With enough spare Enterprise locos, there's no need for them [the list] to go north and can have full logos.
Incicdently, 22036-22039 also have 'half' logos (just the wording)