I've implemented a couple of e-commerce websites with OScommerce, it is an effective solution ... free, simple interface, easy administration, good plugins, compatible with most payment gateways ... I recommend it
you should register a company and yourself as president or ceo of it...
i don't know about taxes in UK, but i think you have to pay some % of your anual income to the govern...
it's the basics of be a legal business
backlinks is the most important part of a SEO Campaign...
I placed a blank web page in the first page of google with a specific keyword... just with backlinks... :bingo: