You can pick and choose which particular shares you want to offset your sells against, you just have to keep adequate accounting records. There is no mandate to use FIFO, the rulings simply say that if adequate records aren't kept then FIFO would be an appropriate method.
See, in particular, TR 96/4 for a detailed description of the accounting records you are required to keep, and TD 33 for the application of those principles to capital gains.
