... to travel Through England to get to Croatia on a connecting flight? I've read the stuff on whether you leave the airport or not. But I'm still uncertain hoe this applies. Has anyone traveled through England and had to transfer planes to get to their destination? What happened? Did you need this ETA?
You haven't provided enough information to answer the question - which airport (England has many) and is your flight a genuine connection (ie is it In one booking/PNR)?
It doesn't matter whether you leave the airport, you need an ETA or visa unless it's an airside connection. Only Heathrow and Manchester offer airside connections.
If you are flying from the United States and you are going to Croatia with a layover at Heathrow and it’s all on the same ticket, then no, you do not need to get the ETA.
Just flew last month from California through LHR to go to Dublin. Yes you need the UK visa thing now. Even for transfers. Be sure to print off your approval. The website said my visa would electronically attached to my passport. When I went to check in at the British Airways counter, they asked to see my approval. Luckily I had kept the email with the approval and showed it to the counter agent. The counter agent suggested we print off the approval email because there have been some glitches getting the visa attached to a person's passport.
Just flew last month from California through LHR to go to Dublin.
Your case was different, you were entering the Common Travel Area between the UK and Ireland, which required you to go through immigration in London, then on to Dublin, so yeah, you needed an ETA.
In the OPs case, if they are in transit, going to the Schengen area, do not go through immigration, then no ETA needed.
Personally, I think I'd get one. What if your plane is cancelled/delayed or worse, and you need to stay overnight in England? Now, most people can get them approved fairly quickly, but my recollection is my husband's took about 14 hours.
"Yes you need the UK visa thing now"
Apologies for nitpicking (sorry!), but an ETA and a visa are two different things. An ETA is issued to holders of visa-waiver countries and is a permission to travel, not a visa.
A visa is issued either because you are a national of a country that is not on the visa-waiver list, or because you need a visa for work/study/living etc. In that case you don't require an ETA.