FlyerTalk Evangelist
Join Date: Aug 2011
Location: Barcelona, London, on a plane
Programs: BA Silver, TK E+, AA PP, Hyatt Globalist, Marriott LT Plat, Hilton Diamond
Posts: 13,047
In general, you simply don't upgrade IB flights using Avios.
You're usually going to find it cheaper to simply buy enough Avios to book the full Avios reward (if you don't have enough), rather than pay an expensive paid fare that allows upgrades.
Or of course you book and hope for a relatively cheap paid upgrade offer.