i have no idea what characteristics would make a soy powder
suitable/unsuitable for an infant, but there are a number of them
which are vegan and sugar-free.  off the top of my head:  natureade
(the most expensive, about 14 USD for ~ 450 g), trader joe's (5 USD for
same quantity), and bread and circus (9 USD).

only the first is likely available in the uk, since the last two are
store brands, but i'm sure there must be similar stores over there
which would carry them. . .

good luck.

janet