The night trip probably has a ton of stops (possible package dropoffs, etc)
Known as a "milk run".
while the other trip was possibly/probably an express train.
PS: when I stayed in London, the cheapest hotel I could find that wasn't completely crappy was for 104 pounds a night. The Hilton by the airport. But I was also being rather seletive about where I stayed (near an underground station). The room was really tiny, too...and the bed was uncomfortable. I think I'd rather stay in the Paris Hilton.
