If you have (or are able to earn) a Gold Upgrade Voucher, you can unlock F redemption availability so long as there is A class revenue availability, which isn't too hard to come by - it generally seems to hover around 2 less than the number of bookable F seats. In combination with an Amex 241, that's how I've managed to book 2 seats in F to SYD for 290k Avios + £2k in TFC, which is a bargain when you consider the equivalent cash fare would have been £15k.