Cost per mille requires only that the publisher make the advertising available on his or her website and display it to the page visitors in order to receive a commission. Pay per click requires one additional step in the conversion process to generate revenue for the publisher: A visitor must not only be made aware of the advertisement but must also click on the advertisement to visit the advertiser's website.
It also shows that you need substantial traffic to make much money through advertising. At a CPM Of £10 with 2 ad units on the site, you would make just £4,000 per month even with a million page views per month for which you serve paid ads to 20% of the audience. Set this to 100% if you are selling all your ad inventory, as for example through Google Adsense.