Bracken is native throughout the temperate & tropical zones, including the UK. It has some limited value to certain insects and birds, but is highly invasive on acid soils. Originally in the UK a woodland plant, it was kept in check by shading. In historic times, as trees were cleared for agriculture, it was controlled by grazing and cutting for various uses (winter bedding, fodder, thatch, and also glass & soap making due to the high potash content of bracken ash). But following the withdrawal of traditional management from large areas of open countryside, bracken has become highly invasive, and now covers 4% of the UK.
If confined to small areas in a mosaic of plant communities, bracken provides cover to a few ground-nesting birds including nightjars, and the dead thatch provides basking sites for adders and egg-laying sites for grass snakes. It can also shade violets and cow-wheat, and thus support several fritillary butterflies. However, if allowed to dominate large areas, bracken completely excludes other plants (and their associated fauna) by shading them out. It produces carcinogenic spores, and toxins which are a potential threat to livestock.
Bracken is OK in the wooded areas round the edges of the common where it is a natural component of the ecosystem. But where it is encroaching on species-rich grassland and heathland areas (especially where some heather is still present) it is controlled by cutting, rolling and spraying with a bracken-specific herbicide (asulam). If we were winning the battle, we would deliberately leave areas of bracken within a mosaic of plant communities, but in fact there is plenty left!