£15 to a company to post a ton of backlinks is probably a sure-fire way to get your site blacklisted in google.
Real SEO, is expensive, but just like any other marketing strategy, you need to balance expense with revenue. I've seen some companies dump hundreds of thousands into SEO to increase sales 1 or 2 %. In my opinion (and if you keep up with google, it's pretty much their mantra) it's best to build for your users first. If you're site is easy to use, has good content, and your users like it, so will SEs.
That said, there are a few things you can do to help improve your SEO performance:
1) Watch your content to code ratio - use Semantic HTML
2) Don't keyword spam.
3) Make sure each page on your site has unique content, a unique title, meta description, and h1 tag.
4) Ask for links from sites that are relevant to your business. A wedding planner shouldn't ask for a link from a lawnmower company website and vice-versa.
5) Don't get hung up on irrelevant metrics like PR, Alexa rank, etc.