HM Revenue & Customs may regard contractual clauses that award a right to the client to dismiss the worker or terminate the contract early as being characteristic of employment. For this reason, it is best to avoid termination notice periods.
However, if an early termination clause cannot be avoided, ensure that you have an equal right to terminate the arrangement as the client, with the same period of notice as the client, so that there is no suggestion of a master-servant relationship.