I’m not a tax expert at all but my understanding is timing matters for things like yearly capital gains allowance, and profit and loss. i.e. if HBomb bought 10 shares of BABA in January at 250 dollars, bought 10 shares at the end of September at 140 dollars, and sold the 10 shares this week at 150 dollars, the shares being sold are the ones bought at 250 dollars. So he’s made a loss, not a profit, right? Had it been the other way around, and the 10 shares sold were the ones bought at the end of September, he’d have made a profit, not a loss. In terms of P&L and capital gains tax and allowance, T212’s approach to LIFO/FIFO is important.
Again, not a tax expert, but that was my understanding.
Edit: according to Richard in this thread, average cost is used in the UK so it makes no difference to your tax and wealth. I’m putting on my ‘I’m not from the UK’ hat and claiming ignorance from you UK folk 
Ignore everything I said. Selecting stocks to sell by price brought