A) Where is Londonderry? I don't have the game up at the moment.
B) You can only build ports or airfields or infrastructure or factories if the province is "in supply", which seems to mean there is one of your units in it or in a province connected to it by land. You can't build anything in Belfast, for example, until you put a unit there. And now I remember where Londonderry is, so that is almost certainly your problem. Send a unit (infantry, militia, garrison, whatever) to Belfast and you will be able to build in Londonderry. I don't remember if basing ships in Belfast will be enough.