LapLap, I bought some tea bags especially for iced green tea on one of my more recent trips to Tokyo. IIRC, I found them in the basement of the Bunkamura Tokyu at one of the separate tea vendors (not in the supermarket bit). The brand is "Ureshino", and they come in a foil package says "Tsumetai Nihoncha". There are 15 bags in a pack, each of which is supposed to make about 1 liter worth of tea, so it might not last you very long.

Which reminds me I should pick up some more, as it's quite good....

Has ItoEn's "Tea'sTea" product not made it to the UK yet? We can get their bottled cold tea here, although I'm sure it's cheaper to make your own. I have a few bottles of the jasmine in the fridge now as Whole Foods has it on sale this week for 4 bottles for $5 (office caf charges $2.50 a bottle, highway robbery....).