* If your turnover is less than £15,000 a year, you only have to submit an overall figure for income/expenses/profits, rather than full accounts. Of course, you should still be keeping full accounts for your own purposes anyway.

* http://www.easyaccountancy.co.uk/about/self_employed_calculator.html has a handy tool for figuring out how much tax you should be paying. I use the monthly field each month to get a rough idea of how much I need to be setting aside for taxes.

* Unlike the US, Britain is very generous in terms of the payment schedule for self-employment taxes: there’s an eight month gap between the end of the financial year and the relevant taxes being due, meaning that if you do a job in April, the tax on it isn’t due for twenty months. If you are doing OK financially and are well organised, the smart thing to do is to figure out the taxes due for each month’s earnings and put them aside in a dedicated savings account. You should then have enough cash in the account to pay the tax bill when it comes due and then pocket the interest.

* As far as I know, National Insurance contributions don’t affect NHS coverage as that is open to everyone. However, the fixed-rate Class 2 contributions are well worth paying because, as you said, they affect pensions as well as some benefits. Note also that while class 2 is voluntary, a self-employed person must pay class 4 contributions which are based on income and are part of the annual salary.