I am in my early 50's and when I was little we bought mix-ups from the corner shop. Some shops had them pre-made in little white bags, other's had a glass display under the serving counter with all the boxes of penny sweets in there and you could point to the ones you wanted up to the value of how much money you had. Some sweets were 4 to the penny, tiny little square jellies a bit like wine gums.
My top favourites were fizzle sticks, cola bottles and chocolate mice.
These shops also usually had jars of the premium sweets on a shelf behind the counter and the shopkeeper would weigh out a quarter of bon bons etc on a large weighing scales and tip them into a brown paper bag.
We usually opted for the mix ups as they felt like better value to us at the time.
My grannny's best friend ran one of these shops in my childhood and she used to let us go behind the counter to the penny sweets and make our own mix-ups - she was very trusting!!
She also sold ice-cream sliders - a block of ice-cream and she would cut off slices with a big knife like an icing palette and sandwich it with wafers. Flavours were banana, raspberry ripple, neopolitan and vanilla. These were a Sunday afternoon treat if one of the grown-ups were feeling generous.
I never heard of or saw pick and mix until the larger multiplex cinemas opened and I was a teen by then