Make sure you stand at the front of the WT area and wave at them. Beckon them to come forward but as they approach the curtain, tell them they can't cross the line!
No, seriously, just say the offer appeared on your booking and you needed the TP to go up to bronze. They will probably stare at you blankly not know what TPs are and perhaps not even know what WT+ is.
I was offered the same price to upgrade before, but it was a return leg from Australia to the UK - and on the then codeshare partner Qantas, so even nicer. And the cherry on that trip was I got upgraded for the first 8 hour sector to Business. It helped that I was travelling along at the end of the xmas break period so a solo traveler has a better chance of upgrade than if in a group. Good luck!