In what order are shares sold?

In what order are shares in a GIA sold? By this I mean if I buy 10 shares of an ETF at £5 each, then later buy 10 shares at £4 each, then later decide to sell £30 worth at £6 each, which of the shares will be sold?
This will impact the capital gains. Is it first in, first out? Or highest gain first? Or something else? What is the strategy?

Is there a plan to allow the user to select the strategy at some point?

As far as HMRC is concerned, I believe the shares are identical so makes no difference:

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I see. The average paid price is taken. Thank you!