My Money is on a battery being past it's best.
As has been said, anything below 2.4 volts, either needs to be charged if it's been stood for a while or a sign it's dyeing.
My guess would be if you read the voltage while hitting the starter, the voltage will likely drop into 10v or under. - eg a battery cell failed.
MDS Batteries have been my go to place online for various batteries for years.
Often cheaper inc postage than the local places are to pick it up.
But I also pick up Motobatt batteries from M&P's as they're local and will price match