Your quoted rate of 10 pounds is about as low as a commission from a discount stockbroker is likely to go, so you are correct in saying that buying 100 pounds worth of stock at a time is not economical.
But, there is a company in the US called Sharebuilder that offers very low commissions and allows you to invest small amounts at a time. They have licensed their concept to HBOS in the UK:
http://www.halifax.co.uk/sharedealing/products /halifax_sharebuilder.asp
commissions are only 1.50 pounds, and you can invest small amounts each month.
Having said that, I would not recommend doing this unless you already have substantial savings and are indeed just dabbling - ie this is money you can afford to lose. Better, in my opinion, to make regular monthly investments into a well-diversified index fund that invests in a basket of world stocks (why restrict yourself to the UK).