"But when I have taken it, it involves getting up in New York very early, and by the time I get home, within London, it's very late, and I'm distinctly knackered to set off for the office at normal time next morning."
Way back we used to use the TWA day time from NY - it was a lot better than overnight - especially in Slave Class but as you say it meant you had to stay close to JFK or get up very early with along drive. Late arrival at LHR wasn't a problem but again meant you had to be staying in London or the Thames valley - both factors impacted useage