You cannot be prevented from taking another job unless your written terms of employment contain a 'restrictive covenant' restraining or limiting your employment for a fixed period of time (usually around six months) after you have left your present employer.
Examples of such restrictions would be working for a direct competitor, or setting up in competition with your employer within a specified geographical area.
Such 'restraint of trade' clauses are only valid in law if they can be shown to be protecting legitimate business interests, are in the public interest and do not unreasonably prevent you from earning a living.
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.