Retail legal advice- help

Hi my name is T, I work in a retail environment in the uk, I have been here for two and a half years and today was the absolute last straw and I am seeking advice or to see if I’m completely unwarranted in my complaints. So I have a basic contract at the establishment as a “retail assistant” but for years now I have been exceeding what my role demands of me. The owner of the business is my bosses brother in law and everyone in the HR department are family of my boss so going to them for advice is not an option, nepotism at its finest, I know. Let’s start with today for instance, today was the busiest day of the month- promotion day. My boss is meant to be opening the store with me at 7am, just before 7 I received a text telling me that she won’t be in at 9:30 instead and has sent someone to open with me who has only been employed by the company for two days. So my jobs when I get in the morning are as follows: Start and pack bakery Fill up any stock that needs replenishing from the stock room Doing a chilled order that comes two days later Make sure all stock rooms and bakery are kept clean and up to standards. My boss, on the busiest day of the month after telling me she will be there at 9:30 (two hours late) to then turn around and tell me at 12:30 that she is not coming Al all is not on. And the reason she didn’t come is the fact she is moving house, which is not my problem, you’re the manager take care of your business. So I’ve had to pick up 4 people’s jobs today and in the middle of it she asks me to do more and when I said I was too busy to complete these tasks she responded with such sarcasm that it jarred me. If I didn’t start her jobs knowing she wasn’t coming in, we and the shop would be in the shit but I’m expected to do it because I like to keep busy. There’s is no one I can talk to as all the hr are her family and I really don’t know what to do, I feel like I need to quit to prove a point but have bills to pay so can’t just uo and leave without finding something new first, I really need some advice. Thank you