The taste of those bottled milk teas vary quite a bit - some I think taste sickly sweet with a dash of detergent, while others are actually quite good.
As for ordering it in restaurants - it seems that in most places if you ask for milk tea, you will be served regular tea with milk (or a side of milk) and sugar sticks/cubes to add to taste. The 100% milk stuff is usually called "royal" milk tea and costs a bit more because of the amount of milk and pain to brew it.
Of course the quality of the tea varies, and there really isn`t much you can do about it as most places don`t really give you the option to order in detail.
You can get some very high quality tea and tea bags at specialty shops. I don`t know about the rest of the country, but
Lupicia is one of my favorites.
But then again, I`m not British so...