England- my workplace hasn't assigned me a payroll number yet, I've been employed there 2 months and they won't pay me until next month

So I started there the 15th of November, I understood the acceptance letter stated that if your employment starts after the 14th of the month you would not be paid until the end of the following month, I was fine with that. Though in training a handful of other got paid and the rest of us didn't despite having the same start date - maybe this should have been a red flag. I have since finished my training and am being mentored. I was due to be paid on the 20th but wasnt. As it turned out I didn't even have a payroll number. This was supposedly due to some missed paper work from the recruitment team. They sent me an email with a form to sign and send back which I did. This was at 10am after my 12 hour shift so I went to bed following this assuming that was the problem and it had been fixed. I then woke up to another email with another form and was told I had 30 minutes to fill it out and send it back. This was followed up with another email stating that I'd missed the deadline and I'd be pain at the end of January. I can't afford my rent, I can't even afford to put petrol in my car to get to work. Is it legal for a company to knowingly not pay an employee for three months? At this rate I'm going to have to quit and get UC so I can at least pay some of my bills. I'm terrified and I can't get a hold of anyone at my workplace to help. What should I do?