Another vote for last flight of the night (though my preference is day flight).

Go to arrivals lounge, have brunch, and you'll be able to enjoy London in the afternoon and evening.

Attempting the 184 without a hotel room to go to at 6am is madness. There is nothing to do at 6am in London. If you do go for 184, make sure you pay for hotel night before / early check-in so you can sleep until lunch time. Basically puts you in the same position as taking the 11pm flight.