Heathrow Airport Re-checking luggage that is not a connecting flight

On separate tickets with checked luggage, this seems extremely risky to me.

You will need to:

• Deplane
• Get to passport control, possibly queue for hours
• Wait for your luggage if by chance you weren’t held up at passport control for too long
• Got through customs
• Get from Terminal 3 to Terminal 5
• Get to the departures area in Terminal 5
• Queue and check-in/drop your bags before the check-in deadline which is 45 minutes before departure for BA short-haul.

After that you still have to go through security (possibly queue for hours) and then to your gate.

In normal times this would probably be feasible if everything went smoothly and your incoming flight was on time, but with very little margin for error: it probably takes about 1h30 out of 2h15 (a bit more or a bit less depending on where you sit in the aircraft, how busy passport control is, the type of passport…).

These days UK airports very quickly saturate any day there’s a bit more traffic than usual, and BA… have quite a few issues, to put it mildly. Queues of several hours for passport control or security, flights cancelled, bags delayed, check-in systems not working are sadly a common occurrence.

I would strongly advise against this connection, especially if you do that on a Friday or Saturday morning or anywhere need some bank holiday or other date with high numbers of passengers. But even in normal times, there’s really very little buffer.

Remember that if you miss the check-in deadline for your second flight, you’ll be considered a no-show, your ticket will most likely be cancelled (possibly including the return leg if there is one), and you’ll have to book and pay for a new flight at last minute prices (I.e. potentially a lot more expensive).

Airside transit (that is, getting to the departure gate without passing Immigration and Customs) is possible at London Heathrow.

However, you describe having checked luggage and two separate tickets — which is inferred from your statement "...on a new ticket for Greece..." — so your transit is more complicated.

With separate tickets and checked luggage, you will have to pass through UK Immigration and Customs in the arrival terminal (UK Immigation, retrieve luggage, then UK Customs), then go to the departure terminal, drop your luggage at the airline check-in desk, pass security in the departure terminal, and go to the departure gate.

In passing UK Immigration and Customs in the arrival terminal, you'll be entering the UK. Depending upon your citizenship, you may need a UK visa to do so. You can check by following this UK.gov link.