This seems to be an active topic for more than just Healeys at the moment.
Not to quibble about annealing, but don't quench the hot metal part in water. This is a practice performed on steel parts to harden them. Copper isn't affected as much by this, but it's not necessary. Let the part cool slowly to be fully soft and if you don't like the way it looks, drag the washer over some wet/dry paper to restore its flatness and appearance.
I buy two different types of sealing washers, both are available on the aisles of places like AdvanceAuto or AutoZone. Nylon drain plug washers are available "plain" or with rigged surfaces. Both work great and are inexpensive. On the Mini I've gone to a bonded metal/rubber seal. This looks like a steel washer with an o-ring bonded in the center hole. You tighten it up until you feel the metal parts touch and you're done. No excessive torque required and no leaks. They're more expensive than the nylon washers but last indefinitely.
Above all else, do not buy and then re-use aluminum drain plug washers. If you must re-use an aluminum drain washer for any reason, dress it flat so there are no raised burrs on it. Failure to use a flat aluminum washer can result in galled threads in the drain pan.