We do use 'soda' in the UK, but it doesn't have the same meaning as in the US. In BrE, 'soda' is a colourless, almost tasteless fizzy drink also known as 'soda water'. It's carbonated water, with a slight alkali taste and only lightly sweetened. 'Scotch and soda', for example, is whisky diluted with this drink. If you said you were buying 9 gallons of soda, we'd presume it was 'soda water'. If you mean that you're buying Coke, Pepsi and so on, you could say 'fizzy drinks' or 'soft drinks'. By the way, we don't use 'gallons' for drink in the UK. Pints for beer and milk, everything else in litres e.g. a two-litre bottle of Coke.