BA uses a different carrier surcharge for ex-US bookings compared to ex-UK/EU bookings.

So a NYC-LHR-NYC award would cost the same avios and taxes, but a higher carrier surcharge leading to a higher cash component compared to a LHR-NYC-LHR award.

JFK-LHR return YQ: US$828.00
LHR-JFK return YQ: US$435.80

JFK-LHR YQ: US$414.00
LHR-JFK YQ: US$217.90

Booking as two one ways should be (414 + 217.90) $613.90 in YQ (with all other taxes/fees being equal) so per passenger that should be a (828 - 613.9) $196.10 saving. Not sure how you got a much larger saving per passenger?
Note: All figures are for J.