I applied for a job as a sandwich maker and was asked to start next day. I worked very hard, but at the end of the day the manager said he didn’t pay people who were working on a trial basis. He did however offer me a job starting in a few weeks’ time. He didn’t tell me I wouldn’t be paid when he asked me to come in.
You are entitled to payment for your day’s work. You could apply to an employment tribunal or pursue a claim in the small claims court to get your money whether or not you take the job. I have to say that anybody who’s prepared to take advantage in this way will probably not make the best of employers.