Thank you for the responses so far, which I need to read again and think about, however:
The key T&Cs of the Lloyds Avios Upgrade Voucher are as follows:
5. Only bookings paid for using full Avios are eligible to be upgraded using the voucher. Avios and Money bookings and bookings paid for fully in cash are excluded.
6. The upgrade voucher will be valid for 12 months from the date of issue. Your booking must be made within the 12 month period, after which time the voucher will expire and will not be replaced or extended.
7. Upgrade vouchers can be used to upgrade either 2 one way flights or 1 return journey booked on a British Airways main line service. British Airways codeshare flights are excluded. Flights from London City airport are excluded.
8. Using an upgrade voucher enables customers to upgrade their booking by one cabin class. Customers can upgrade a booking to the class above as follows:
from Euro Traveller to Club Europe;
from World Traveller to World Traveller Plus; or
from World Traveller Plus to Club World.
Bookings cannot be upgraded to a First Class Cabin.
9. Airline taxes, fees and charges or Reward Flight Saver (RFS) cost if applicable must be paid on all tickets booked. Customers who upgrade their journey using the upgrade voucher will have to pay the increase in airline taxes, fees and charges or increased RFS cost applicable to the cabin that the customer upgrades to.
On the basis you can pick up sale fares to NY for around £1500, this makes sense. 1.35 x 80,000 plus £500 of taxes is as near as matters £1500.
If you include the avios earnt on the revenue ticket, the avios value will fall much closer to 1p.
For option 3 a flight booked in WT+ (Blue membership level) earns 4323 (LHR-JFK), presumably per direction. In addition the costs of £1005.85 would earn 1257 Avios at the 1.25 rate. Therefore this option costs 10097 Avios net. Assuming a CW fare was available at £1500 this would value 1 Avios at 4.89p. But you do have to spend over a £1000 for the ticket in the first place, which is not an insignificant sum.
I wouldn't worry about the connections per se, you would be rebooked if you ticketed yourself on one PNR, but of course the more connections the greater the risk - certainly of delay, maybe of downgrade, irrespective of airline.
As noted by others adding the UK leg onto the same ticket would negate a lot of the savings in terms of charges, and I don't think in this case you would easily be rebooked, but what are the implications of missing the connection - would you forfeit the Avios and cash paid? Even arranging to arrive the night before may not be enough security as there could be weather delays etc. outside the traveller's control.