There are thought to be more than 15,000 different fungi in the UK and many of them are very difficult to identify so, although some are edible, it is unwise to try any unless you are an expert.
Most fungi grow underground or in rotting wood where they spread unseen. The 'mushrooms' we see are the fruiting bodies which they throw out periodically and from which the spores are dispersed. Some fungi can be found all year round but Autumn is the best time to look as most appear then.