if i look at the taxes/fees/charges difference for 2 adults and 1 child for LHR-JFK CW out and WTP back compared to CW both ways i get around £390 total difference, so your figure does seem high. where are you getting it from? sometimes the online uua tool can give an incorrect figure.

LHR-JFK CW out and WTP back
661.66 x 3 = £1,984.98
Air Passenger Duty - United Kingdom £185.00
Passenger Service Charge - United Kingdom £56.06
Passenger Civil Aviation Security Service Fee - USA £4.30
International Transportation Tax - USA £15.00
International Transportation Tax - USA £15.00
Animal & Plant Health User Fee (Aphis) - USA £3.00
Immigration User Fee - USA £5.30 Customs User Fee - USA £4.60
Passenger Facility Charge - £3.40
Carrier imposed charge £370.00

LHR-JFK CW out and CW back
791.66 x 3 = £2,374.98
Air Passenger Duty - United Kingdom £185.00
Passenger Service Charge - United Kingdom £56.06
Passenger Civil Aviation Security Service Fee - USA £4.30
International Transportation Tax - USA £15.00
International Transportation Tax - USA £15.00
Animal & Plant Health User Fee (Aphis) - USA £3.00
Immigration User Fee - USA £5.30
Customs User Fee - USA £4.60
Passenger Facility Charge - £3.40
Carrier imposed charge £500.00