As an ex-pat who lives on English tea, the only one tea that tastes good in the US (maybe it's the water) is the M&S Extra Strong. Haven't seen them in the T3 M&S. If the shops in T3/5 don't stock it you might be able to contact them on line and ask to get an order delivered to whatever terminal you want.
However, coming into the US there's no problem with customs - had a random search of my case last time and they were amused by the number of M&S tea bags I had in my carry on
