Battery volts at 12.3 standing are a little low but entirely normal on a battery after a few years. My own car with 7 year old battery was down to similar value after a week or so without use and of course started fine.
I would thoroughly dry car/engine with hair dryer after wash, charge battery and try again after plugs have had time to dry out as well.
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Dont think he made it clear whether it would not start before or after the wash?
