There are 24 hour coaches to the airports - and a taxi to the airport is also often cheaper than to central London.
There’s 24h buses and trains to St P, and 24h tubes on some nights too. But you don’t need them to get to St P for 0645.
You can get a direct train to St P from Bedford, Cambridge, Peterborough, Brighton, Corby, Derby, Ramsgate, Gillingham, Ashford, Sutton, Horsham, and most intermediate stations, plus of course all the underground directly or with one change. With one change or a short walk from Euston you have connections from almost the entire southeastern and Southern suburban networks, a decent part of the South Western suburban, all of the Elizabeth Line, much of the London Overground, and the WCML from Northampton southwards.
It is not hard, and not at all ridiculous.