When people are looking for the best SEO firms in their local area, my advice is always to simply search Google for them
Search engine optimization is, by definition, about understanding how to create and manage websites that perform well in search rankings.
If you run a local search for something like "SEO agency London" the organic results that you come across should be the ones that have earned Google's approval, and so should be the ones that you can trust too.
Although not necessarily completely untrustworthy, I tend to ignore the agencies that have advertised to show on the first page for a term like that, mainly because I'd be interested in working with people that have cracked SEO not PPC...
