Are there any special signs for passengers arriving from Ireland?

I am confused with the CTA but no common visa policy. At least with Schengen visas you know that you're not breaking any rules travelling between member countries but how could passengers know whether you're supposed to wander around Heathrow looking for an immigration officer or whether you can just exit. When I disembark I follow the signs 'exit' presuming the country I arrived at will route arriving passenger in a manner so that they are screened according to its requirements...

Perhaps someone will be able to advise on a specific situation? I will arrive at Heathrow as a visa national and clear immigration to spend a day in London. Go to Dublin the next day for 2 days (clearing passport control at DUB as well I presume?), then come back to London for a day on the way back to KBP. Will I need to 'enter' (clear passport control) the UK after I have arrived from DUB?