It is well to remember (though I am sure most users are aware) that Yorkshire tea and "Irish Tea" etc. ,are just blends of various black teas...as is "Russian Caravan" tea I used to get from Twining. They are often quite nice tea (I had some Yorkshire tea given by some nice guests from England a couple years ago), but I really love to choose and blend my own when I can.
It is getting harder & harder to find shops here (Wash. DC area) which carry the good loose teas, partially due to the extreme pressure of the powerful coffee companies and advertising. Tea is a bit more exacting to make properly, and folks want **convenience**.

I can, with a little driving, obtain 'almost' any teas I care for...and if I want to spend real money, there are online sites to order the special 'estate' teas from India & China...etc. So far, I make do with just 'decent' Darjeeling, Keemun, Assam...etc...along with some generic Indian teas and various Twining blends.

As to relative strength and potency, tea is always a matter of balancing the amount of leaves used with steeping time....and the fineness of the tea.. The finer (smaller) the leaves, the stronger the brew.
In tea bags, what is often used is "D, FNGS, BPS or CTC" and related names. This has nothing to do with type of tea, but just refers to the size and the part of the plant the leaves come from.

It is usually assumed by tea marketers that those who use tea bags are not expecting higher grade teas, and those "D, FNGS, BPS or CTC" grades are used, with moderate variation by better companies.. (sadly, the best known American tea is Lipton's, which, as far as I can tell, uses some of the cheapest pickings)(Sir Thoma Lipton was never in the tea business directly....his family were soap manufacturers, but his name was nice, and was bought to look good on tea packages).

Tea, like most products, require sorting thru the advertising claims, and learning what is truth and what is hype, in order to make a REAL choice.

(I still will never put milk in tea....I still 'suspect' that the practice was begun to disguise bad, cheap tea. ☺