El Cid,
Nothing to prevent anyone earning money in any way, no. But the Inland Revenue do have the power to define them as an employee, not self employed, and demand they pay tax and NI accordingly. The definition of which is which is quite complex and I'm not an expert, but in general to have them classify you as self employed you do have to work for more than one person/company, use your own equipment and/or premises, and be the one who decides on hours of work, what you charge etc. Individual tax offices will interpret borderline cases differently, and you may be able to argue the case - though in my albeit limited experience you'll probably lose.
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Whirly
To fly is human, to hover, divine.