Dumb question - have found 2 x A fare First class seats LHR-SYD. Can I use GUF2 + BAPP Companion voucher without any I class availability to redeem as Avios booking? Also related question if this was for a cash booking, can I book at cheapest Business class fare available for the same ticket - i.e. £11K business vs £16K first cash?

thanks
JRC