As others have said you can UUA BA Holidays bookings. You can check for availability online and then call. If no availability and you want to fly those dates no matter, then make your BA Holidays booking and continue checking until availability opens up and then call to upgrade.
See also:
http://www.flyertalk.com/forum/briti...day-avios.html
http://www.flyertalk.com/forum/briti...s-booking.html
http://www.flyertalk.com/forum/briti...-holidays.html
http://www.flyertalk.com/forum/briti...l-booking.html