When you started did you sign anything that opted you out of the working time regulations ( which came from the EU) You are not expected to work more than 48 hours a week- averaged over a 17 week period unless you have opted out. The gov.uk website gives some good advice about it here.
Working time regs. If they are breaching that then they are actually acting unlawfully. Join Usdaw - they are the retail workers union. Acas will help individuals with contract issues so you could give them a call 0300 123 1100.
If the three of you take out a collective formal grievance against your manager they would have to consider it. have a look at the company grievance and discipline policy. Write everything down. Every hour you do extra without pay, who told you to stay late/start early, what was the reason for it.
Happy to help further if you need me to. Just drop me a message