There are two ways to use Avios. The traditional (?) way is to book a reward flight using Avios via Avios.com. Availability is non-existent for business class, in practice, on transatlantic routes but is available for economy.

Option 2 is to buy a cash flight as normal on the EI website and use Avios as part or full payment. There is no issue with availability as it’s a normal cash booking BUT the Avios needed are usually higher than for a traditional reward booking and so it is not a great option unless you have lots of Avios to spare.