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. Without a contract, you risk not getting paid, losing your artworks, being unable to prove what you have sold or given away or even losing money of your own.
If the agreement you are entering into is complex, you are also strongly advised to seek professional legal assistance in the country you will be working in.
Contracts are simply written agreements, signed by the parties making the agreement, so that all parties have a copy of what is being agreed, when it will be done by, when it will be finished, and what money is involved. Verbal contracts are as binding as written contracts, but have the disadvantage of being legally unprovable as to their existence.
Contracts must follow the laws of the country in which they are to be valid. Contracts should mention whether or not they are international, and if so in what countries, and if so which legal system they will be covered by.