The Gerber Puffs(if that is what you mean)would be very expensive in the UK due to importation costs
You get a lot per container though and they've got minimal sugar(1 gram to every 60 pieces in at least one flavor),so it is a matter of figuring out how much you value them(as the cost makes them far more than you'd ever pay in the US).
Actual puffed rice however should be available just about anywhere in a grocery store,probably around cereal products.Just plain puffed rice is all you need with those,nothing with any dyes or added sugar or preservatives.