Connecting flight in Heathrow, will we need to go thru E.E.S and security?

It would appear that both flights are BA. Are they on the same ticket? If so, then you stay airside and wont go through Passport Control, thus you wont need an ETA. But you will probably go through security. Just follow the Connecting Flights signs. The EES is the European entry and exit system. You will encounter this on arrival in Venice before passing through Passport Control. Customs is for goods you are bringing in. 99.9% of tourists just pass through the Nothing to Declare door.

EES is for Schengen. The UK is not in Schengen. You will go through EES in Venice.

You will not have to go through immigration or customs at Heathrow. Follow the purple Flight Connection signs to go between flights.

You will go through security at Heathrow.

If your flights are all on one ticket you can use Flight Connections and not enter the UK officially. Just follow the purple signs and not immigration. You will need to go through security. EES is a Schengen system and has nothing to do with the UK. You will go through EES, immigration to enter Italy and customs in Venice.

I was wondering the same thing since I fly from Bologna to LHR, then on to DFW. The answer I got was that if my bag was checked through to DFW, I wouldn't be required to clear customs in London.

I just did this in June, coming the other way, from Amsterdam to London, then to Portland, OR on a through ticket, you will need to get lined up for security. It was quite a line, but if you get delayed and it's getting too close to your flight time, talk kindly to someone; they were good about helping people get through the line. Just pack your patience and keep a smile on your face; it's all good, you are on vacation!

As others have mentioned, the UK is no longer in the EU/Schengen countries, so they have their own protocol for screening incoming flights and going through flight connections/security. You won't have to go through UK Immigration, since you are staying 'Airside' at the airport. 2 hours 45 minutes should be fine, but if you get delayed, it may change that.

Once you land in Venice you will be subject to the EU controls and the EES Schengen protocol, when I landed in Venice last October, I had to go through the immigration/passport gate for non-EU passports, scanned my passport, takes my photo and I believe my fingerprint, but I can't remember if that was part of it or not, then a quick look at my passport by a border official, that was my first trip into the Schengen region since they had implemented the EES system of collecting electronic data.

Thank you all for your clear explanations. I appreciate all the specifics.