I really can't see that they would do what you are suggesting......

I'm lucky that I can put work expenses on personal credit cards, so have no issue in reaching the £10K within a few months of the start of the year.

I have seen on this very forum that there is the possibility to buy a fully flexible plane ticket, get the voucher, and then cancel the ticket

I think I also saw that there may be costs involved in cancelling a fully flexible ticket.

Would another, more honest, way to do this be to buy vouchers for the store where you do your regular shop and use those over the coming months whilst earning the miles/voucher upfront? If you have a Waitrose near you then you can even get a 2.5% discount (at least!) off the face vaue of John Lewis vouchers.