Wait for the next I-class fare sale out of the UK, and buy a round trip or open jaw with the LHR-EWR/JFK in I class. Play around with the return leg, e.g. with NYC/BOS-LHR/LGW/DUB options booked on a much farther date in the future than your cruise trip, and at the cheapest economy fare compatible with the I fare you can find. It will likely end up costing you 100-200 quid more than half the LHR-NYC-LHR round-trip I fare.