Where in north London? What day of the week?
Thameslink is supposed to operate hourly overnight, if you live near to a station
First tube departs Hammersmith at 0528
Night tube runs on weekend on Central and Victoria
Plenty of night buses
Drive and park in Hatton for free (won't be any traffic at 0500); 10 minute walk plus 15 minute free bus to T3
If no bags, aim for 0630; if bags aim for 0600