As I stated before the airports in London very from central London. Thankfully there are options of getting there like a cab, the train, or the cheapest option coach buses.
There are different coach bus companies like easy bus, national express, and many others who not only go to the airport but to other parts of Europe. When going to the airport you will notice there are different prices depending on what time you go to the airport. Please do not try to book the cheaper bus which is 2hrs away from your time because some of the drivers will not let you on. My friends and I booked a 4:15am bus, we arrived at London Victoria Coach station at 3:45 am. We tried to get a 4am bus and he didn’t let us on even though there were seats on the bus. The ticket says you are allowed a 60 min grace period in either direction. This never happened to me before and I have taken so many buses.
It’s hard to pick a time that works especially when you do not know how long border control will keep you. The only advice I can offer is to estimate everything close so this doesn’t happen to you!