Legal documents and template contracts to purchase online, including contracts with galleries.
As an artist, you have specific and well-established legal rights, as well as general protection under the law on things like employment or getting paid.
Copyright is international through the Berne Union for the Protection of Literary and Artistic Property (or Berne Convention for short) which does not refer to the laws of any country in particular.
Droit de Suite, or Artists Resale Right, came into force in the UK in 2006.
Before getting involved in any employment, commissioning, selling or other work in any country, including the UK, it is highly advisable you ask for a contract stating your rights and what you can expect to get from your employer.
As well as entitling you to respect and, hopefully, a wage, professionalism also includes a commitment to getting a job done once you take it on.