This is often cripplingly high. I mean, if you're just putting £100 on, you'll wave goodbye to £10 for each trade you make. Even if you just buy one stock and sell it one year later that's £20 gone plus any capital gains taxes.
I know the stock market is for people making long term investments with big lump sums, but for people at the bottom, the barrier to entry is too high
http://en.wikipedia.org/wiki/Dividend_reinvestment_plan#Cash...
Most brokers also have some range of mutual funds with no trading fee.
Building up funds over time is probably a better use of small amounts than trying to earn percentages trading them. For one thing, even an amazing annual return on £100 isn't going to cover a whole lot of your time.