I concur that if you can get a BA Amex and trigger a 2-4-1 voucher that is the way go. You basically double the value of your miles...

If you are going to take the plunge and do that you need to know how quickly you will be able to trigger the voucher. I assume that as this is a honeymoon that your dates are fairly fixed.

For fixed dates next summer I'd be wanting to lock these flights in the next couple of months which means charging £10k to the card quickly!

You need to work out if you are going to get through the spend quickly enough.

If you are brave and disciplined enough (and have spare or accessible cash to pay off the credit card!) you could charge a fair chunk of the £10k on some fully flexible flight tickets to trigger the voucher and then refund the tickets once you have legitimately spent the £10k.