You ask about getting the visa the easy way... there is no easy way. Bureaucracy exists everywhere and the Thai's are not novices when it comes to red tape.
My advice to you is to goggle Thai Sole Proprietorship and see some of the hoops you will have to jump through. Just about everything is on a case-by-case basis.
Also read the "Thailand Foreign Business Act B.E. 2542"
Realize you cannot perform any restricted occupation. You will need to register the company, you will need a business visa, you will need a work permit, and you will have to pay both Thai taxes and foreign earned income taxes to Great Britain. The costs can become prohibitive. It may take you a considerable amount of time to build adequate clientele to pay the bills and become "profitable". You will need capital seed money.
In your shoes I would consult with an English speaking Thai legal service to review the prospects and to get a formal quote for setting up the business.
Good luck. Nothing worth having comes cheap or easy.
you need to set up a company, if you do not invest hundreds of millions of THB or have a special technology, the only way will be to keep it 51% thai owned. So you need some partner, investor, lawyer, who you can trust to hold the 51%.
when I started up a found a lot of information(on setting up companies and workpermit and others) on the website winvasia(you can google it). They also have a directory of hundreds of lawyers, accountants, webdesigners,...