How to preserve tea taste for 6-7 hours?
Have you considered investing in a good (read: expensive) insulated flask that is rated for 6+ hours and hot liquids? As long as you only open it long enough to pour your cup out, it should keep it at a drinkable temperature and (hopefully) not spoil the taste like reheating does.

If you do this, you should heat your milk separately so it doesn't cool down the poured tea.