Species new to Britain are not as uncommon as you may think.  WIth more and more people becoming interested in fungi and field mycology there are more hands and eyes to unearth things.  It’s the experts/taxonomists that are thin on the ground and it is them that will be able to tell whether what you have found is common, rare or species new to Britain. With experience the slight differences between various species become more and more apparent leading you to the conclusion that what you have is not usual.  One then has to follow the steps of checking literature against your micro and macro details, which is often not at one’s fingertips, at which point it has to be handed over to the experts.