Layovers in London

From JFK to Manchester via LHR you will go through IMMIGRATION/PASSPORT CONTROL at LHR. You walk through green door for Customs, nothing to declare.

From Croatia to JFK via LHR you will not go through PASSPORT CONTROL at LHR as you are not visiting the UK.

Thanks for the info! Would an 1h 40 min be enough time?

So on the layover in Gatwick, no customs/passport control? Just gate transfer?

I'd be more comfortable with more time than the layovers you mentioned, especially the first one after such a long flight from a busy airport like EWR into busy LHR. I'd allow more time on the ground both ways, just in case.

I don't know whether you've considered Icelandair, but they fly between EWR and Manchester, skipping LHR on your way over, and between LGW and EWR home. You have to change in Reykjavik but it's generally easy even with short layovers at that small airport. Of course you'd have to get yourself back from Dubrovnik to LGW (or some other European hub) on another airline.

Implied but not crystal clear (and with a potential short layover at LHR, I think you want to be certain):

Both flights are British Airways both legs.

All on a single ticket?

For the DBV-LGW-JFK trip BA shows a layover of 2+ hrs at minimum ( for a random date in Sept)

As BA is selling it, presumably it is enough time. As a single ticket the onus is on BA to get you to your destination

Here is the Heathrow airport connection website. Put in all the details of your flights, and you'll see all the steps and the estimated time it will take: https://www.heathrow.com/flight-connections

I haven't flown through Gatwick, but I've gone through LHR more than once. There's a cursory security check when you get off the plane, but the line can take a few minutes. Then you walk a bit and go through passport control (another potential line), then another security check, very very very thorough, which can mean another wait in line. I don't go through passport control since I am in transit to another country, so I'm a little hazy on that. Then you have to get to your gate. Terminal 5 is big. You'll see overhead signs with the time it takes to get to your departure area. It can be 20 minutes. The website Harold posted claims it takes 60 minutes, using a 6.15 arrival and 7.55 departure. I assume that is from the arrival area to the departure area. Remember that a 6.15 landing time is not being in the terminal at 6.15. Even if your plane gets to the gate at 6.15, it takes a few minutes to attach the sleeve and start getting passengers off the plane. If you aren't in first or business or priority coach (or whatever they call it now), you'll be among the many people waiting for all those people sitting in front of you to move their butts. Remember that a 7.55 departure is not the same as being at the departure gate at 7.55. In my experience gates close at least 10-15 minutes before departure. It's possible that you won't have any landing delays or lines in the airport. According to the website, you only need to go through customs if you have something to declare. On your way home, you go through passport control in Europe and passport control and customs in NY.

Be sure to get your boarding passes for your connecting flights when you check in for the first leg. If you have checked luggage, ask for the bags to be tagged "short connection" and check that the correct airport code is on the tags.

I just flew to LHR a couple of weeks ago and the line to get through passport control took 1 hour 15 minutes...and that's not the longest I've ever taken at that airport. I've flown into LHR 3 times this year and the fastest time I got through was about 40 minutes. The fastest I've gone through was 15 minutes but that was such an anomaly! Neither of the layover times you have listed are even close to long enough...I never allow less than 3 hours for Heathrow and usually try to make it closer to 4. If your flight into Heathrow is at all late you need a good amount of buffer time. Gatwick is a little better than LHR but not by much...you need more than 1 hour 20 mins.

Thank you for everyone's input! My flights are on single tickets, but it looks like there are some flights with longer layovers. I found flights with a layover in Heathrow just under 4 hours on the way there.

Then on the way back, I found a flight with a 2h 5m layover. So a bit more time there too.

Thanks! It also looks like I won't have to worry about passport check in Gatwick since my bags will be checked through.

International to international flight connections
If you’re arriving at Gatwick from an international flight, are connecting on to another international destination you have some choices to make depending on your circumstances:
1. If your hold bags are through checked or you’re travelling with hand luggage only and you have your boarding pass, simply follow signs for Flight Connections*. There is no need to clear Passport Control or Customs.