Not quite - unless you explicitly state that the service must be given personally by the employee. And I've never heard of such a clause being part of a work contract.
A work contract, or to give it its proper title, a "job and person specification" is a list of tasks, and their outcomes, the incumbent is expected to accomplish. How they are accomplished is in most cases up to the person tasked with achieving that outcome. If the job includes cleaning the toilets, then my only expectation in that regard is clean toilets. If the employee then goes and finds someone to clean toilets in his behalf at a lower rate, what boots it as long as the job gets done? Of course, if the substitute hired to actually clean toilets then goes and cleans out the company safe, the employee who hired them has to bear their share of the responsibility for that.
It's like subletting a rented house. Here in Australia, the law forbids a landlord from preventing a tenant from subletting rooms in the house to other tenants. The tenant has to notify the landlord of course, but the landlord cannot stop the tenant from doing so. But if the sublet tenants then knock holes in the walls, it's the original tenant who has to foot the bill, because they're the ones who signed the contract with the landlord.
So if an employee outsources all or part of their job, as long as the company security is maintained and policies and procedures respected, then there's no problem. To punish an employee for simply finding a cheaper solution when that is what the entire company is about doing, is nothing more than sheer mean-mindedness.