Cutting of lines at the till point

Went to Spar Gateway during my lunch hour today and only picked up 2 items. Due to it being my lunch hour and rushing to get back to work on time I searched for the till point with the least people which I finally found - there was 1 person at the till paying for their items and another in front of me, the person in front of me then stepped aside to answer their phone but was watching their place in the line, I did not step forward as I knew this person is next. A lady or should i say uncivilized mannerless individual with a trolley full of goods just goes infront of me without asking if I am in the line and just starts offloading her trolley at which point I politely aproached her and told her that there is a line - this lady then gave me a load of attitude which broke out into an argument which I eventually walked away from and have been very upset about since. Your cashiers surely can see who is in their line to pay at their till yet they say or do nothing about this rude disgusting behaviour. After this very unpleasant and upsetting experience at this store, I am really thinking twice about shopping here again - not to mention that my family and I are regular customers here.