Does the UKBA earnings rule apply to the gross or net income?

The UKBA (U.K.Border Agency) has just brought in a rule saying you can't bring into the UK, and live with, a spouse unless you earn over £18,000. My question is, does this apply to the gross or net income of a self employed person?

Basically, a self employed person can claim far more expenses than an employed person. For example, the cost of running a car, buying a laptop and materials - the tools of his/her trade. These costs are offset against tax. Which means a poorly paid s.e. person with a good accountant can often in real terms be doing better than an employed person on better gross pay, who has to pay tax and N.I. at source and for such items out of taxed income. But in minimising their taxable income so, does it mean they fall below the threshold of the UKBA bar?

For example. A s.e.person earns £30,000 gross (ie. turnover) but after expenses such as his car are deducted he declares a net profit of £10,000 so he pays little tax. Does the UKBA look on his earnings as £30k or £10k? The former would allow him to bring in his spouse, the latter not?

Does anyone know the answer?

Thanks.