London international transfer - how much time to allow for separate tickets?

On separate tickets give yourself a minimum time of 4 hours to make the London to Crete flight. Just about all airlines allow you to check in print boarding pass 24 hours in advance. Easy Jet allows 30 days in advance.

Coming back Athens to London on a separate ticket the best practice is to fly in the day before your flight back to the USA and stay near the airport so you lessen the risk of missing the flight and having to buy a very expensive one way ticket.

Whatever you do, use the same London airport for arrival and departure. People make a huge mistake when they have to get from, say, Heathrow to Gatwick, to make a same day connection.

This advice from Wally deserves emphasis.

I also agree with the 4 hr minimum advice.

Also take note some airlines close boarding 30 minutes prior to departure, some when looking at the time between flights you account for that.

I too emphasis "Whatever you do, use the same London airport for arrival and departure.", and if that airport is Heathrow, be aware that Heathrow is effectively 3 airports. Terminals 2+3, Terminal 4 and Terminal 5 are 3 separate locations round the airport. Allow an extra hour if changing terminals at Heathrow, you have to take the public train.

Gatwick has 2 terminals (North and South) with a driverless train transfer between, add 15 minutes for this.

If at all possible, do your check-in for the second flight, and get your "boarding card" before leaving home. Otherwise, even with hand luggage, you will need to go through immigration (passports) to get to the check-in desk.
If you have hand luggage, AND your boarding pass, you can stay airside.

If you tick all the above boxes, 4 hours minimum is probably OK. If any boxes not ticked, add extra time.

If it's possible in your itinerary, I'd allow a day or two in between flights and check out a few sights in London before catching a flight back to Miami.

You have enough information here to make a decision, but I would add one more piece of information. At Heathrow, you may land on time but you may not have an open gate. That means a wait on the tarmac.

We did this once and made our connection with time to spare, but I worried from the time I realized (thank you, forum) that this could be a problem until we made it through security for our flight at LHR. We were able to get our 2nd boarding pass at our home airport, but even though booked through the same airline, the 2nd flight was a codeshare and they wouldn’t transfer our luggage. If you have carry-on and the boarding pass for the 2nd flight, you can follow the purple “flight connection” signs at LHR.

We do separate tickets through Heathrow a lot, because we use miles to fly British Airways in business class to London, and then fly onward on tickets I purchase later, when I have figured out where we are going.

For the Europe-bound flight, we allow the 4 hours suggested by others before our onward flight. For the homebound flight, we always fly to London a day or more ahead; I do not want to risk missing that long-haul flight. We love London and are always happy to have some time there.

We have done that a number of times but always allow a min of four hours if not more. But two trips back we were bit. It was into London and easyJet to Scotland. Unfortunately, our original leg from the US was cancelled and we were delayed three days in being rescheduled. So, we lost the cheap easyJet ticket and had to buy a new one at full price. That is your chief risk.

Wally said

Personally I would stay overnight in London in both directions to avoid a same day connection problem.

That's what I would do: at least one night each direction to minimize problems that could become very expensive.

Personally I would stay overnight in London in both directions to avoid a same day connection problem.

I agree, although, on occasion I have risked it on the way to my destination by building in at least 5 hours in between flights. However, even if your flights go off as scheduled, it makes for a terribly long day at the airport. I would strongly advise flying back to London the day before to help mitigate any flight delays due to weather, strikes or random airline carrier sloppiness. Remember, your main international carrier doesn’t owe you a thing if you cannot make your flight home on account of delays etc. and the cost to rebook that one way ticket home would be astronomical.

For separate tickets, I allow a 4 hour cushion for layovers at Heathrow.

So, we don’t do separate tickets. Nice that 4 hours seems to be the recommended cushion but it’s all anecdote. If you miss your plane you miss your plane. Calculate what that would cost, add that amount to your budget and figure out f you could handle it. Bottom line: there is no “safe” time as stuff happens.

Thank you everyone for the great information, especially about Heathrow airport! Right now I'm planning RT Gatwick with 6-8 hour layover and on the return flying back to London the night before, spending the night and leave Gatwick the next morning.

I travel to London each month for business from the U.S. There were several trips last year where I did exactly as you plan. Generally, I had a 3-4 hour connection time. I always carried on/had hand luggage. A few times I had to connect through different terminals. Every time it went smoothly and only took 30 minutes max to connect, including bus transfers. Heathrow is one of the best airports. Enjoy!