Do I go through customs in London on the way to Italy?

If it is a single booking, the airline has figured out how long the stop over/ transfer time needs to be. If you are booking the trip as two separate flights, anything less than 4 hours could be hazardous to your wealth.

If all you are doing is changing planes on a single ticket you do not go through customs, but there will be a security screening between flights.

No, customs and immigration will be handled in Italy so nothing like that will be needed in London.

The UK however always requires you to go through an extra security check during transfer.

I'm guessing you're talking about Heathrow....this link should help:

Heathrow Flight Connections

Immigration and customs are separate processes. You will go through immigration in London. This is where they check your passport, ask you the purpose of your visit, how long you are staying, etc. Then you go through security again before boarding your connecting flight. Heathrow is huge, and this process can take a lot of time. I would want 3 hours minimum connection time.

Once you get to Italy, you go through customs. This is where they are concerned with the items you are bringing into the country: fresh fruit, meat, large quantities of cash.

You will NOT go through immigration in London if transiting on to Italy.

You will follow the Flight Connection signs and go through security again. Airline reps may check your passport and documents to make sure they are in order for Italy.

Assuming this is one one British Airways ticket, you will NOT, as Frank said, go through immigration/passport control at Heathrow. You will transit “airside” and technically you do not “enter” the UK. You will go through security before reaching your departure gate, even if the transit is within the same terminal (T5). However, as Frank II mentioned, the gate agent may need to check your documents again to be sure you have what is necessary to enter Italy. But it is more likely these will be checked in Seattle before you board your first flight.

We have flown BA from Seattle to Italy, Spain, Switzerland, and Germany (all pre-covid). I like to allow at least 2 hours between the flights for getting through Heathrow without stress, even when the flights are on one ticket. This is more than the “minimum connect time” required, but I prefer to have that. In some 8 such trips, we have only needed the extra time once, but I still like to do it that way for peace of mind.

Again, you will NOT go through immigration in London because London is not a part of the Schengen zone. Travel4fun is in error. That would be the situation IF changing planes within the Schengen zone. In the Schengen zone you go through immigration at the first airport you land and customs at your final airport.

This all assumes you are booked on one ticket from Seattle to Rome/Milan (where ever you are going). And you are just changing planes in London. IF you have two separate/independent tickets -- one to London and one to Italy from London --- that is a very different issue.