If you only drive the forklift in areas which are not classed as roads, then you can drive any forklift, even without a current class 1 licence, as long as you have an appropriate qualification such as a forklift operator’s certificate or equivalent.
However, when you want to drive a forklift on a public road (and remember that the definition of a road is very broad), then you will need a class 1 car licence if the forklift is up to 18,000kg (including the load being carried) and a class 2 heavy vehicle licence if the forklift is over 18,000kg, plus an F endorsement and operator’s certificate (here’s what you need to do to get a class 2 licence).
In reality, the main areas where a driver needs a class 2 licence plus F endorsement when driving a forklift is for work on wharves and ports, and in railway yards lifting empty containers. While this is often done using a top lift container handler using a gantry spreader which requires a wheels (W) endorsement, sometimes forklifts are used because twenty-foot ISO shipping containers have two slots at the base for use with a forklift.
Even forklifts that only lift empty containers will have a nominal capacity of 7000-9000kg, meaning the gross vehicle mass will far exceed 18,000kg.