Calling all experts! Upgrade advice!

Hello!
I'll start with a bit of an introduction! I've been using the forum for a while now, reading the posts daily and really enjoy hearing about all of you who are so experienced in your travel! The trip reports are also so interesting! So thank you very much for all your advice and comments.
I do need some advice....(nice lead in....lol ii) )
I am flying to JFK on 24th Sept on VS45 currently on a K class ticket. I want to upgrade this outbound sector to UC (I want to experience the LHR CH for the first time!) At the moment my booking is quoting a £761 PP to upgrade.....
So what would you do if it was you?
Book now at the £761 pp, guaranteeing the seats or wait until closer to the time to see if they come down OR even wait until arrival at LHR?
Help.....
I'll start with a bit of an introduction! I've been using the forum for a while now, reading the posts daily and really enjoy hearing about all of you who are so experienced in your travel! The trip reports are also so interesting! So thank you very much for all your advice and comments.
I do need some advice....(nice lead in....lol ii) )
I am flying to JFK on 24th Sept on VS45 currently on a K class ticket. I want to upgrade this outbound sector to UC (I want to experience the LHR CH for the first time!) At the moment my booking is quoting a £761 PP to upgrade.....
So what would you do if it was you?
Book now at the £761 pp, guaranteeing the seats or wait until closer to the time to see if they come down OR even wait until arrival at LHR?
Help.....
