Dividend reinvestment programs often offer the opportunity to purchase shares with no fee (I guess usually after an initial purchase):

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.