I've heard two versions of the union story that prevented code shares in Belfast before. One says no more than 2 destinations per airline (there are some airlines that have more, but they're all part of the AF/KL group); the other says no more than one code share airline per country ex Amsterdam.
Whichever is true still applies as EI offer ORK and DUB and FlyBe have codeshares from Amsterdam to UK destinations.
I believe it's the former. The Dutch pilots' union is not really interested in geographical markets, more in preventing KLM-coded flying from being carried out by third parties. That means limiting the number of destinations.
For example, I think you'll find that flyBE are limited to 2 UK codeshare destinations to AMS (
INV and SOU). Given that they fly to AMS from more than 2 UK points you could imagine that it would be a win/win for both BE and KL if that codeshare could be extended.