There's one fancy loose leaf brand called the English Tea Company that is extortionate so I can't recommend them in good conscience but their tea is nice and fresh and the range is crazy.
Otherwise I always find apothecaries have got the best teas, at the best prices normally. Especially if you like the strong fragrant black teas, I had some really nice imported stuff from Japan, a Chinese tea but grown with Japanese methods and it was the best I ever had. Trouble is I finished it on the day I opened it during a long Xmas sesh and now the name is gone.
Another great option for cheap tea is to make friends with health food shop attendants, buy some cheese and leave with a half kilo of tea to boot