Assuming your ticket would be the most expensive economy ticket, you would only earn about 14k avios for the return trip. You could by that many through the groupon thing (might now be closed) for a little over £100, which is about the value I'd place on that many avios.

If you were due to fly in Y on Delta and were offered an upgrade to J for £50 a leg, what would you say? If the answer is "yes" (and surely it is), then you would be mad to take the BA flight.