Gentle YTA. I'd shelve the holiday for now and try to work it out so that at some point in the future, you can both go together in first class. I get that you're upset about the wedding issue, but that's another can of worms which you need to address, if it's become a contentious thing for you.

I know how expensive first-class flights are when you're booking long-haul from the UK. I did one once with Virgin many years ago, when they were offering first class for an extra £600 on a flight to the USA - they were upgrading their cabins at the time and this was a plane that hadn't had the upgrade. It was fantastic and well worth the money to me, but at the time I had a lot of disposable income!

I'd also recommend to do the first class flight on the return journey, so you have something to look forward to at the end of the holiday.