I just noticed I missed out the key point of the question. CTA to UK domestic or other CTA e.g. DUB-LHR-JER or DUB-LHR-NCL
If you landed in LHR from DUB and were off on a domestic flight and didn’t know what to do (but did pay attention to the signs) would they direct you in this case to the FCC forcing a needless passport check or out via customs but bypassing a passport check or is there a third option I’m not aware of?