Hi. OK. Sorry to rain on your parade ... but it doesn't matter where your bank account is, or where your customers are, etc. If you are in Thailand for more than 15 days and you are working then you are legally required to have a work permit (and appropriate visa). And it doesn't sound like you would eligible for a work permit based on the work you propose to do ... unless you want to set up a company, employ staff, etc. Of course many proceed regardless ... up to you
No-one can really advise you as to what YOU need to do with visas as there are a few avenues and you need to investigate which one is the best fit for you (or least worst fit). Start with the Thai embassy web site in the UK and also perhaps google on "thai ministry foreign affairs". Happy reading. Ed visa would be the place to start and most likely route forward.
If you are living in thailand for more than 6 months then you are a tax resident of thailand and are meant to lodge a tax return (though many don't). As to whether you do this and what you tell the UK authorities ... this is something you need to talk to an accountant about ... it depends on your long term plans, whether you end up getting a Thailand work permit, your personal financial circumstances in the UK, and other variables.
