It would be virtually impossible to define the most successful website design company in UK markets. You’d have to ask individual clients about their satisfaction with the service they received and you’d have to create some kind of extraordinarily complex metrics to test the SEO success of the sites built. Plus, you’d need to interact with the site’s end users to find out how satisfactory they found the site – on PCs, laptops, mobile devices and TVs.
Clearly, one definition of a successful design company in any sphere is simply the number of people, who continue to use its services. We can, for example, say with some confidence that a website design company, whose customers keep using it rather than jumping ship and going to someone else, is a company, whose services are appreciated by the first end user; that is, the company or person commissioning the site design.
We can also say, with an equally reasonable amount of confidence, that websites that enjoy a regularly large number of visitors, whose calls to action are routinely taken up by enough people that profits continue to rise, are successful enough. Though here it must be noted that success is a relative term: is, for example, a site that ticks over a decent profit as successful as one that makes less actual money but enjoys more regular visitors?