Wow thanks for the replies guys, I wasn't expecting this. Much appreciated.
So the general consensus seems to be it would be hard to upgrade by 2 classes and if possible it would cost £600 more.
I think the draw to WT+ is that the seats seem to be more spacious which I want for an almost 10hr flight. If I get to the airport and it's available for <£200 then I will go for it (pay in cash).