It is not unusual to see a written term in the contract of employment limiting your employment by a competitor, or setting up on your own in competition, for a limited period and within a limited area.
Such 'restraint of trade' clauses will only be upheld in law if they can be shown to be reasonable (i.e. in duration and scope) in terms of protecting your employer's legitimate business interests and to be in the public interest.
The wider the scope of such a clause, the less likely it is to be enforced. Where a clause has been drafted too broadly, the courts will normally find it entirely invalid rather than accept part of it or rewrite it.
This kind of term is most likely to be found in employment contracts where the role involves sale or marketing, or in the contracts of senior employees who are in regular contact with the employer’s clients.