Having a quick look on National Express Liverpool-London route for 13th/14th October (i'm not making the trip just intrigued what the fares are & comparing it to Flixbus), fares for the daytime coaches are £9.40 single, were as the 01.45 service it's £28.40 which is a bit steep, most journey's now take around 7 hours.
By contrast Flixbus for the overnight service is £20.49 for the 13th or £22.49, for the 14th, with day fares ranging from £6.49 to £11.99, most journeys take around 6 hours.
Both companies have a stop in Birmingham on the Liverpool-London route, National Express goes through Manchester which adds an extra hour to the service.
If i was making the trip by coach, i'd use Flixbus now, & won't consider National Express anymore.