Traveling through Heathrow

The best place to check is the UK Government website - there may be an important personal bit of information needed (that probably shouldn't be revealed) that is not the same for someone giving their experience.

If you do need a visa of any sort it depends as to whether you are going through Border control or not and your airline can advise on that. The 48 hours is crucial - you don't have to stay that long, but there is a different visa (if you need one) if you do.

Normally you would have an immigration check on arrival in the middle country. Depending on the terminal it is normally just before a security check and tends to be fairly quick.

What country is your passport? If Canadian, you do not require a visa for UK. Nor Italy, but eventually there will be a need to complete the Schengen ETAIS documentation...not a visa, but a registration.it keeps getting deferred, and I think at last look it was 2024.

You haven’t told us your citizenship. That is key to any answer to your question.

You should be able to follow the signs for flight connections on your return journey and not need to go through arrivals. You will always go through security.

US passport flying from Vancouver BC

I see now from the UK govt website, as of today, no need for a visa.

Correct. No need for US citizens to get tourist visas in UK.

For your return flight from Milan to Vancouver, assuming you are simply changing planes at Heathrow, you will not go through immigration. You will stay “airside” so technically do not enter the UK. Simply follow the purple “flight connections” signs for the terminal for your next flight. You will go through security but it is quick. Make sure you have no water in your water bottle if you carry one, and pull the 3-1-1 bag with liquids and your tablet, Kindle, or laptop if you have one, out of your suitcase for the screening.

Lola's reply assumes that your flights from Italy to Vancouver are all on one booking? If it is that's fine, but if you have booked the Italy flights separately from the transatlantic ones you may have to go through immigration, collect any bags and re-check for your second flight. There's no problem with that as long as the first flight is on time and you have left plenty of time to do everything.

Thank you everyone for your patience and information.