Skipness,
I understand your point about JFK-LCY LHR-JFK but, if I may say so, I'm convinced you're wrong!
The low hassle factor (15 min check-in time afore departure) of LCY and the complete removal of the immigration hassle factor at JFK (performed at Shannon with your fellow 31 - at most - other pax, are worth their weights in gold.
Last time I went to the US from T4 - yes I know T5 is now magic but more airlines fly the pond than BAW - but it took me 40 mins in Fastrack to get through security; last time I went through LCY there was no queue and two smiling sets of searchers and friskers were in attendance.
Arrive at JFK and just walk to the taxi rank - bliss, well until you get in the cab anyway!
It'll work.
Saman