How to make softer scones?
That recipe has a very old style problem - it predates modern definitions of "cup". Methinks the cup in your recipe is a small coffee cup.

If you are using 80g (3oz) of butter, that will make a good scone recipe with about 225g (8oz) of flour and about 150ml (5 fl.oz) of milk.

I just weighed a 250mL cup of SR flour, it contained about 155g.  The cup in the recipe is a "not quite full" 5oz coffee cup according to my calculations.

There's your trouble ... too much flour and milk. The fact that the adjusted amount of milk needed in proportion to the butter is very close to the customary ratios reinforces the points I have made. Another clue is that the recipe says "makes 16". If you get a lot more than the recipe says, that should ring an alarm bell.