I also have a Credit Card that I *only* use for making small purchases like DVD's or CD's that I buy online from well established companies.
I have set my credit limit to £250.
I only use it because buying by credit card offers you more protection if the purchase is over £100 like you mention.
I don't incur ANY interest charges as I pay it off in FULL every month, so it is like an interest free loan.
I think it is very wise to use a credit card when buying online especially with all the phishing and fraud that's going on now, as at least you will have SOME protection in the event of something going wrong!