The departure tax is generally high anywhere in the UK, with LHR being a bit more.

That being said, LHR will definitely give you the most options on DL. You can fly thrice daily to JFK and they are usually open. You can also fly to ATL, DTW, MSP, BOS, SEA, and PHL.

MAN is hit or miss in my experience. Sometimes its a lightly loaded 767 and other times its an oversold 757. Keep on eye on MAN if you want to go there.

EDI is another option but it tends to be a bit full.

ZED on VS is a fantastic option. It opens up a lot more destinations and they are great to ZED with. I just rode them from LGW - MCO and it was a breeze. You check-in online 24 hours before departure and they assign seats right then and there. No need to talk to a single agent until they scan your boarding pass at the gate. VS operates all over the US from LHR, LGW, MAN, and GLA.

PM me if you need more info! Happy to help.