probably obvious question re upgrading S or W to G

I know I should know the answer (and it's probbaly a really obvious answer like 'phone FC') to this but am suffering from brain ache.
I'm looking to use miles to upgrade from PE to UC - 40k miles
I know I need to be in S or W (S preferable to save a few pennies as its £ 1280 compared to £ 1975 for LHR to NYC for when I want to travel)
(If there was a sale I'd probbaly go for a Z but no sale at the moment)
I know I also need G availability but where in the booking engine process does this show up?
I got as far as the page to enter my CC details but obviously don't want to fork out if there are no G's (and yes I was logged in and used the option to select an S fare).
I'm looking to use miles to upgrade from PE to UC - 40k miles
I know I need to be in S or W (S preferable to save a few pennies as its £ 1280 compared to £ 1975 for LHR to NYC for when I want to travel)
(If there was a sale I'd probbaly go for a Z but no sale at the moment)
I know I also need G availability but where in the booking engine process does this show up?
I got as far as the page to enter my CC details but obviously don't want to fork out if there are no G's (and yes I was logged in and used the option to select an S fare).