Enter your expected annual profit and see how much you would actually take home either way, worked out from this year's real HMRC rates.
It runs your profit figure through the same Income Tax, National Insurance, Corporation Tax and dividend tax rules SoleDirector itself uses to calculate real customers' tax bills, not a simplified rule of thumb. Figures update automatically whenever HMRC's rates change for a new tax year.
Not always, and not by a fixed amount. It depends on your profit level, and thresholds and dividend tax rates change most years, so a rule of thumb from a previous tax year can be wrong today. Run your own number above rather than relying on general advice.
Use the second field above. A sole trader's full profit is personal income the moment it's earned, whether spent, saved, or left untouched, so it makes no difference there. A limited company only pays personal tax on what's actually drawn out as salary or dividends; profit left inside the company is taxed once, at Corporation Tax, and stays untaxed personally until a future year it's drawn. That's the real advantage of incorporating for a business that doesn't need to spend everything it makes straight away.
Yes. Many people start as a sole trader while profit is low, since it is simpler to set up and run, then incorporate once the tax saving outweighs the extra admin. There is no penalty for switching, though it is worth timing it around your tax year.