You can sell any parcel of shares you choose as long as you keep records to ensure you don't sell it again 'twice' down the track - i.e. you can't avoid tax forever!.
Each share is considered a different CGT asset for tax purposes - people purely use first in first out for simplicity (including many of their tax advisers).
The link tells you that each share is a different asset - but from experience any parcel of shares can be sold in any order to your own best advantage.