You need to find a good indian friend who's parents can send the chai masala to you and then make masala chai yourself at home . SE1 isn't great for good authentic indian cuisine and although ASDA, Tesco's etc do stock lots of world foods, you don't always find what you are used to. There are a few asian stalls at Borough Market who MAY sell the chai masala. Or you could try grinding the spices together yourself!
Not SE1, but I was surprised to get it from a little shop on a platform at Baker Street station once! I guess it made sense, given the selection of samosa, pakora etc on sale alongside the sandwiches and sausage rolls.