I'm sure the 21:30 JFK -LHR flight will be a sleeper service, its normally just the first couple of flights that leave around 6PM that aren't sleeper.

I would recommend taking one of the 747 flights and try and grab the UD or grab a couple of middle seats downstairs. If you are taking CW out to NY then you will experience the full service then on the way back you can do the pre-dine in the lounge, have a spa treatment and then when you arrive in LHR you can take full advantage of the arrivals lounge. If you fly during the day or via LCY then you wont have this.

You can still eat in the lounge and then grab dinner on the plane (its a smaller menu) however would recommend skipping breakfast on the plane and enjoy the lounge.

Chessy