LHR connection

Hi. Perhaps you have read the information from Heathrow? If not, this is what they say for Transfers:

If you have a boarding pass issued for your onward journey, you will be able to stay airside during your connection. This means that when you arrive into Heathrow, you will not need to pass through immigration or customs and can follow the purple signs for connections and make your way to the departing gate once it opens.

My understanding is that with that connection you will be put aside
after going thru customs so that there will not be a quarantine
requirement when landing in Italy.

Your understanding is wrong. You will need to quarantine upon arrival in Italy if you have been in the UK for the last fortnight. Even if you have only been there for an hour. https://www.salute.gov.it/portale/nuovocoronavirus/dettaglioContenutiNuovoCoronavirus.jsp?lingua=english&id=5412&area=nuovoCoronavirus&menu=vuoto&tab=6

Badger is correct - you will have to isolate for 5 days if you enter Italy via the U.K. plus show a negative Covid test. This policy is valid until 30 July and from what I can see, no information is available for what happens in August onwards. Having regard to the rising U.K. infection rate, I can’t see this policy being reversed at present.

“You will be put aside after going through customs. . .”

Just to clarify: the operative word is “airside”, not “aside”, and you do not go through “customs” at LHR.

Transiting “airside” means you do not go through passport control/immigration (what is commonly but incorrectly called “going through customs”). You transfer directly from one flight to another within the airport secure zone and do not go to the arrivals area or baggage claim. At Heathrow, you still go through a security check before you reach the boarding area for your onward flight.

Differently countries have been regarding this airside transfer differently with respect to covid and quarantine requirements, and the situation is changing because of the increasing spread of the Delta variant. So you need to understand the current requirements for the country you are visiting. For Italy, the current requirement is a 5-day quarantine after a flight from the UK, even if you have stayed airside. You still have been in the airport and potentially exposed to the virus. So you need to have a plan for this if flying to Italy, at the current time.

Thank you for all the helpful replies. We are going to change our outgoing flight so we connect in JFK. That will make it much easier.