FYI 30k a year and £400 a day are type of prices that are speculatively thrown about by recruiters and agencies and are not really the market rate for qualified, senior software engineers.

A senior software consultant in a large UK city is billed out at around £150+VAT an hour or £1200. Contracts can be quite long (in some cases practically infinite), and fairly easy to set up once you know how. Typically you need to have some experience from the tender process at an established consultancy firm.

If you are doing this as a one-person consultancy you are responsible for all taxes and benefits. As a very general rule of thumb you can take about half of what you earn out of the company as salary, the rest will be lost on taxes and expenses. Alternatively you can take out a lower salary and dump excess capital into funds, property or any other appropriate investment.

You are at the mercy of your customers, and generally they can get rid of you at short notice, although in practice its no less secure than any other private sector job.

£150 and hour is £6000 a week. Typically you can budget with a working year of about 42 weeks (52 weeks in the year minus actual holidays, 8 bank holidays, sick days, training/conferences). This equates to £250,000 a year in revenue which will generate a maximum salary of about £125,000. A decent amount, but pretty much in line with what you could expect to earn at an established software PRODUCT company in a major UK city.

Consultancies tend to pay out about 30-40% of their maximum billable rate for software engineers, so if you were working for a company with these type of rates you would expect a salary of £40-50K