The day flight is doable if you live in the NYC area, but is iffy if you have to connect at JFK (I misconnected some years ago on a DCA-JFK-LHR trip and spent 12 hours in the JFK area; thank goodness Aqueduct was running that eay.

On a second try a few years later, Mrs. ND76 and I successfully made this connection, but it was an extremely long day. We left home at 4 am to make the 6 am departure from DCA to JFK. We got to JFK a few minutes early and got to spend 45 minutes in the VS Clubhouse, where they had breakfast but no adult beverages (we were there before 8 am). The flight got off the ground just before 9 am but didn't arrive at LHR until 2030 local time. It took us something like 90 minutes to clear immigration, then we took the tube to our hotel in central London, arriving there at 2345 or so.

The UK Home Office has improved the immigration queue significantly with the introduction of automated passport gates for US citizens (among others), so you should get through immigration within 20-30 minutes of stepping off the airplane (still a long walk from the gates Delta uses), and the Elizabeth line train is vastly superior to the Tube.

I have gotten good at sleeping on planes, and I can get a free tasty breakfast at the VS Revivals Lounge in LHR T3, so going on a redeye to LHR is fine by me.