If you have identified merchants for whom you think you could generate substantial revenue through an affiliate marketing relationship, there’s no reason not to attempt to set up a direct relationship. There’s also not much magic to pursuing this type of arrangement; if there isn’t a pre-existing relationship, start by reaching out to your potential partner and telling them a bit about your site.
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.