Not sure about putting water from the teapot back into the kettle to reboil - after all, the kettle might next be used to boil water for something that won't benefit from a slight undertone of tea. I usually put slightly more water than is required to boil and take a bit off to warm the teapot just before the boiling completes. By the time you've swilled out the pot and put some tea in (personally, good quality tea bags are nearly as good as loose leaf and somewhat more convenient), the boiling is finished and you have near 100 degree water to get the brewing going.
Milk first. Pasteurised, preferably non-homogenised (but well-shaken), full cream or semi-skimmed to taste but absolutely not skimmed, and definitely not UHT. Just a little, though adding more can help make poor quality tea more palatable.
No sugar.
And never* wash the teapot.
The only place where I don't do this is at work where speed and convenience mean bag-in-cup does the job, and lack of fellow tea-drinkers means UHT milk is the only practical solution.
Scum on top of the tea is usually because the water you've boiled is "hard". Around here that is never an issue (some of the softest water in the country - don't even have to put salt in the dishwasher). Not sure if there'd be a benefit to pre-softening water used for tea. I've never lived in a place with water hard enough that I felt it necessary to experiment.
M.
*ok, so perhaps you can wash it once a year or so, but give it a good swill with boiling water afterwards and don't expect the first couple of brews to be quite up to scratch. If bits of stuff start flaking off the inside when you pour the tea, this is absolutely not a problem if you are already using a tea strainer. Oh, and make sure you empty it and swill it out and leave it with the lid off if you're going away for more than a few days. The last thing you need to find when you get back, desperate for a cuppa after a long trip, is that your teapot is rapidly evolving new lifeforms.