It depends on your budget, but a return from MAN-JFK in W can be quite competitive, often cheaper than from LHR. You could then use your avios to upgrade to Club World. Or a Y flight out to JFK and then a W return, again, upgrading your inbound to Club World.

Alternatively, if you want the absolute lowest cost option, your avios could book you some really decent accommodation in Manhattan. While you wouldn't be realising the best value/avios, its a way to certainly bring the cost of the trip right down.