I have just returned from a return journey on Flixbus N1328 from London to Warsaw West. It runs once per day. The full single journey takes around 26 hours. On way out it departs from London at 09:30, and on way back it arrives in London at 18:45. Hence the Dover to Calais ferry crossings are during the day, and the overnight elements are through Germany, albeit with a passport check at the German / Polish border. Timetable here:
FlixBus
bustimes.org
P&O Ferries on way out. Irish Ferries on way back. I had a previous disastrous FlixBus trip from London one Easter, when our coach got repeatedly turned away at Dover Port, then gave up and returned to London. My advice is to not make international coach trips during school holidays - tell me if you disagree.
I believe this is the longest FlixBus route from the UK. I was attracted by the simplicity; just sit in your seat and no need to worry about missing connections. I did actually alight at Łódź on my outward journey - a really attractive, vibrant city and had great weather. I got the train on to Warsaw - just buying a turn up and go ticket - and stayed for two nights in Warsaw.
I was the only passenger who travelled the full route. Passengers boarding in London typically alighted in Belgium / Netherlands. Passengers boarding in Poland typically alighted in Germany. And there were intermediate Belgium / Netherlands to Germany passengers. Seats get reused during the journey. So while I'm making a 26-hour journey that includes an overnight element, others (e.g. London to Belgium / Netherlands) are making a daytime journey, and some of the Germany passengers are boarding late at night but might only be on board for 2 hours. Noise levels were fine though and the lights were off at nighttime.
The coach was a Scania / Higer Touring three-axle. Generous legroom, reclining seats, plug sockets, centre sunken toilet which was clean and stocked with toilet roll. The hand basin water and WiFi didn't work on way out but did work on way back.
I knew before travelling that you can change at Warsaw West for FlixBus routes to Ukraine. I noticed two separate Ukrainian mums and kids on my outward coach. While in Warsaw, I went to see a FlixBus departure to Ukraine. This seemed novel to me, but at Warsaw West bus station I found that Ukraine is possibly the most popular destination country, with several operators heading there include one with a former Megabus right hand drive Van Hool 'interdeck' blue coach. The bus station was full of adverts in Ukrainian.
My journey was on time on way out, delayed by one hour on way back due to German border quizzing Ukrainian people.