British Airways offers convenient, albeit not very well communicated, twilight bag drop service at London Heathrow or Gatwick and Edinburgh airports.
Passengers traveling before 1 PM in Heathrow and Gatwick can check in their bags between 4 PM – 10 PM the night before. The hours are even better in Edinburgh.
You can access BA’s page for this service here.
Here’s the information from BA:
Overnight Bag Drop is also available from 16:00-22:00 for flights departing the next day before 13:00 from London Heathrow T5 or London Gatwick. At Edinburgh it is available daily from 04:00-20:00 (04:00-18:00 on Saturdays) for flights departing before 09:30 the following day.
This overnight service, some call it twilight, is very convenient for those flying out early in the morning and would like to drop their bag(s) before heading to one of the airport hotels for the night.
Not sure how busy the bag drop stations in the morning are, as I always travel with just carry-on luggage, but this may alleviate any possible wait and allows for sleeping a bit later.