Hi Amelia,
The primary ones to consider are Outbrain and Taboola - Zemanta is good as well but works a bit differently. nRelate recently shut down. There are others out there, but Outbrain and Taboola will likely be the best option for business-focused content. In my own research, the other networks tend to focus more on B2C topics. Ryan also mentioned Skyword and Contently, which are software companies that offer marketplaces to hire writers, and Curata, which is software to curate content socially. All 3 offer good solutions, but don't fit into the discovery platform description I believe you're looking for.
For content discovery platforms, you'll probably pay in the range of 10-50 cents per visit. However, the only real targeting they're providing is geographic, and other targeting is controlled using their algorithms.
Because of this, I'd suggest you consider paid social as an alternative solution. You have a well-targeted group of individuals - UK business owners and landlords. Other networks like Facebook and LinkedIn will offer you much better content targeting with the same intended effect - drive traffic from your core audience.
You may pay a bit more per click depending on content performance, but the benefit of targeting makes it worth it. We've seen performance as low as 2 cents per engagement on Facebook for popular content, and CPCs as low as $1-5 for highly targeted audiences (for example, CEOs in the US who make $100k+ and are interested in very specific industry topics). If my audience is CEOs, I would much rather get 500 visits from CEOs for $500-1000 than get 5000 visits from random individuals in the UK reading business articles at the same cost.