Do note that the Avios required for upgrade shown on Qcalculator is the lowest amount you'll need assuming there is still award availability at normal redemption rate, which you can then upgrade through your booking management page. If you are upgrading at the airport, it will be much higher than what's shown on Qcalculator (and there is a separate page showing the amount required for airport upgrades).

Also, you will not have lounge access with J tickets upgraded from Y. If you are getting the J award ticket outright (which is 75k Avios one way for UK to BKK on normal redemption rate), you will have lounge access.