As far as I can see on ba.com, the YQ on JFK - LHR (or vice versa) for J is hugely greater (~80% greater) than the YQ for WT+, i.e. $414 each way vs. $229. Therefore the UUA is likely to be an extra £120, not "in the region of £60-70".
Is there any way for you to request a quote before confirming the upgrade?