Thank you for your insight on my question. Based on that and knowing my own stress level limits - I did some more checking and was able to find a flight a day earlier - LHR-MAD (so no mad dash to LGW) and arriving in Crete 36 hours earlier than original flight - which is nice!https://www.heathrow.com/transport-and-directions/travel-between-airports gives you some info. Looks like at least an hour to do it and it isn't particularly cheap. You will also need to factor navigating the airports at either end. Immigration and customs at Heathrow (frequently a mess) and security at Gatwick. I would probably leave at least 5-6 hours honestly especially if I was on separate tickets. Actually I'd think long and hard about doing this at all. Also if you are doing this anytime around Brexit (end of March 2019) I would even think a lot harder.