Some of the best bonsai trees in Japan can be over 200 years old. If it's worth £100 I would guess that it would be at least 10-15 years old.

Bonsai trees aren't meant to grow fast... if you want something fast growing, just plant a regular tree and then trim it as it grows to make it into a bonsai. By pruning the tree while it is very small, you can train it to make a large number of branches in a very small area.