The answer, as usual, is "it depends".
Essentially, you'll have to pay VAT on your turnover, and if you have a million turnover, you'll have to be VAT registered. In that case, you'll either charge VAT on top of the million, or you'll have to send 200k to HMRC straight away. OTOH, being VAT registered means you can offset the VAT on company expenditure against the VAT on the company's income.
After that, you'll deduct all allowable expenditure from the income of the company. That doesn't include your living expenses (those are your problem, not the company's) but basically anything that makes the company tick. Company expenditure also includes salaries paid to the company's employees etc.
The company will then pay corporation tax on the remainder.
Oh, and once all that's done and you get some dividends from the company on top of your salary, you get to pay income tax on those, once it becomes your money. But the money in the company isn't yours, it's the company's. Big difference.