Found some restrictions on BA's website:

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Spend My Miles
Check availability and make reservations from 3 to 355 days in advance of today's date.

Save miles when you book online before 15th April - save 5000 miles on award tickets from North America to London and 2000 miles from London to other British Airways destinations.

If you are member of a household account, wish to book a more complex journey, purchase a Mileage Booster or book a partner, upgrade or companion award, then please call the Executive Club direct.

Contact the Executive Club on: 1-800-452-1201