What are the uses of brake fluid on a lorry?

Those typically don't use brake fluid - they normally use air brakes. Can't really speak for the UK, but in the US, trucks with a Gross Vehicle Weight Rating of 26,000 lbs. or less tend to use hydraulic brakes, while vehicles heavier than that use air brakes. However, many of these vehicles with either manual or automated manual transmissions do use a hydraulic assist clutch, and brake fluid is what's used in those.