Article 4 of the European Convention on Human Rights
Article 4 – Prohibition of slavery and forced labour

1. No one shall be held in slavery or servitude.
2. No one shall be required to perform forced or compulsory labour.
3. For the purpose of this article the term “forced or compulsory labour” shall not include:

• any work required to be done in the ordinary course of detention imposed according to the provisions of Article 5 of this Convention or during conditional release from such detention
• any service of a military character or, in case of conscientious
  objectors in countries where they are recognised, service exacted instead of compulsory military service
• any service exacted in case of an emergency or calamity threatening the life or well-being of the community, or
• any work or service which forms part of normal civic obligations.

There are at least 27 million slaves worldwide. That’s roughly the combined population of Australia and New Zealand.
In the UK the official figure is between 4,000 to 4,500 but that is probably a lot higher as many slaves are kept undetected.