There are many kinds of trees. Do you know the name of the tree that is the topic of your question? If not, post a picture of it and a closeup of a leaf or two and some of its berries and the GOY members will identify it for you and this will help in getting you an answer.

I don't know where in the UK you are but it could be the lack of rain. Is the ground very dry? If so try giving the tree a good bucket of water.

welcome to GoY and it is possibly lack of water or it could be what is called 'june drop' when a tree will shed some of its fruits/berries as it cant manage to keep them all.
if it is lack of water then you will need to keep watering it for several days especially if no rain is forecast.