Very sorry that you had this experience but yes NEVER consider a job offer finalized until a contract has been drafted and both parties sign. Until then it's all basically vaporware
P.S. Not doubting your experience, just comparing it with mine.
Employers must to provide you with the fine print within two months of your start date:
https://www.gov.uk/employment-contracts-and-conditions/writt...
Any incontrovertible evidence of an agreement of the job, verbal or written, counts as a contract. Everything else is just finalizing the terms and conditions, which either party can agree to amend at any later date and which many employers assert they can do unilaterally.