Insect and Spider Identification: Very strange behaviour of mystery spider?, 2 by Mysteryspider

Mysteryspider wrote:
Hi, I am in the UK and have a spider infestation in my garden. They are 3-5+cm, dark brown to almost black. They havd a lighter shade of browny green on their front. The females are brown, because there\'s one with a egg sac attatched to under her abdomen. They are EXTREMELY fast. Ive noticed that even when unthreatened, some stand on their tiptoes. One has faced me directly, pointed at me with its left front leg and fangs and shook violently. What on earth is the spider and why did it do that??