Requirement for IELTS and Work in the UK by: IELTS buddy
It's not necessarily a requirement for every job. It's up to the employer to decide that.
If the job requires you to speak English then the employer will need to know your level, and IELTS will provide this. So they would then make it a requirement.
For example you would need an IELTS score to be a Doctor here (7.5) but you would not need IELTS if you were coming over to work on a farm and pick vegetables.
Immigration may make it a requirement in cases as well, regardless of the employer.