A Jumping Spider (family Salticidae) found in Britain.   It has caught a fly, which can be seen along the bottom edge of the image.   The British Jumping spiders are around 5-6mm in length.

There are 4000 species of Jumping spiders in the world, of which only 4 occur in Britain.  Jumping spiders are carnivores and tend to be most active during the day, particularly in warm weather.  They have 8 eyes which give them 360 degree all-around vision.  The four largest eyes are in a row on the front of the head, the two central eyes being the largest and most adaptable, allowing focussing, sideways, up and down as well as rotational motion to home in on their prey or a mate.  Having detected a suitable small animal to catch, they analyse it with their remarkable front pair of eyes.  They creep up slowly until just a few centimeters away, before making a powerful jump and catching it with their front pair of legs.  Although they will tackle large insects, they are seldom successful in catching anything much bigger than themselves.  Jumping spiders are generally found at ground level, but sometimes occur in trees where they are quite capable of jumping from twig to twig.  Females guard their young as they develop inside a specially woven silk pouch, but do not feed them.