The algorithms used by Google to determine the relevance of a website are secret; therefore, the SEO companies are not able to know them perfectly. However, the most reputed companies offering SEO services in London know the main characteristics of those programs, therefore they would be able to create websites that respect the majority of requests of those search engines.
It seems like the number of links pointing to a website is considered important by Google, therefore your website must have high link popularity. This popularity is obtained by posting links from other websites to your page, and by commenting in blogs and forums. There is also the method of directory submission, and social media marketing, so as you can see, the process of link building is not as easy.
With the help of SEO companies, you will be able to benefit of a solid link building chain that would increase the general popularity of your site. This company uses only White Hat SEO techniques that are liked by Google, avoiding the possibility for your website to be penalized by search engines for using illegal practices.