Sounds exactly the same as I had a few weeks ago. I got caught in ridiculous rain, car ran fine, parked up for a couple of hours, drove for about 30 mins then every warning light came on. RAC said battery wasn't charging, they lent me a spare to get me home.
I bought a new battery and checked with a multimeter to see if would charge under load, so aircon, stereo, lights and stereo on. It appeared to be fine and I check it every week or so. I put it down to temporary dampness, but I'm still keeping a spare battery in the boot for now.
My advice is buy a new battery and multimeter before you go steaming in getting the alternator or voltage regulator changed. It's a massive job to get to the alternator, so spending £70 on a battery first isn't going to go to waste. Charge the old battery and keep it in boot for a while, just in case you aren't lucky enough for the fault to be a one off gremlin.
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