I’ve had no real problem redeeming long haul in J or F, including using companion vouchers, but you do have to be flexible over travel dates and organised enough to look a long time in advance. If you want to travel on Christmas Eve to JNB in a family of four, you are going to be sorely disappointed. But I’ve already got my 2016 holidays to Brazil and the Cayman Islands sorted, in J, using Avios. So it can be done.
The reason they will be inflexible about the number of reward seats and upgrades on a flight, even when there is capacity, is it results in cashless profits. The provision for BAEC balances goes down, resulting in income in the P&L, but minimal cash is received.