There are around 670 species of spider in Britain, 3,000 in Europe, and 48,000 that have so far been discovered so far, and all of them play an important roll in a balanced ecosystem. Whether they help keep numbers of pest species down of other invertebrates like flies, mites and lice by eating them, or whether they are eaten themselves by birds or small mammals, they play a very important role in keeping a healthy environment. Species are arranged in their respective families below.
For further reference see below:
Britain’s Spiders A Field Guide by Bee, Oxford & Smith is an excellent reference book.
Collins Field Guide Spiders Britain and Northern Europe by Michael J. Roberts is also worth a look.