Can't comment on the specifics of T1 as I've only ever arrived there from outside the CTA, but if Irish and Channel Islands flights arrive in the same place as international ones then I can't imagine there not being, at the very least, an invisible Customs presence - especially for Channel Islands flight, seeing the Channel Islands are outside the EU.
It's seperate from the international area - completely. A different floor in a different part of the building. I don't believe any Jersey flights go from/arrive at that area (or from LHR at all right now).