The Four F's is an informal term and mnemonic in English common law for fruit, fungi, flowers and foliage.
People can gather the Four F's so long as it is for personal use, and not for sale or commercial gain. This does not mean that people can enter land unlawfully to do so, but in areas where they can lawfully be, for example on a country park, or walking along a right of way, they are entitled to collect and take away the Four F's.
Like most common law this provision does not apply if some other legal provision over-rides it, for example the species in question is specially protected, say by listing in Schedule 8 of the Wildlife and Countryside Act 1981. The same act also prohibits uprooting of any wild plant without the landowner's permission, so the Four F's only applies to picking, for example daisies in a public park; not to digging up, for example, bluebell bulbs or young trees.
Legal challenges to this right are unusual, and not always successful.