Dobbo - the only flights I can see leaving Europe around 6 pm to go to NYC are on aircraft operated by a European airline departing from one of their very large hubs with all the benefits of feeder traffic. Thus Air France form Paris, KLM from Amsterdam, Scandinavian from Copenhagen, Swiss from Zurich, etc...
The only proper exceptions to this rule in all of Europe seems to be at London which has the advantage of linking the commercial capitals of North America and Europe with all the senior management of major global corporations travelling back and forth willing to pay big bucks for a flight ticket. London has almost 30 flights per day to NYC - yes almost 10 times as many as Manchester
If British Airways were to base several or more long-haul aircraft in Manchester *and* re-open a hub at MAN, an early evening flight to NY might be feasible. Until then, daily 6 pm flights from Manchester to the USA would certainly be popular with Mancunians but terrible on profitability and are thus looking really quite unlikely to happen.