Volunteering for Cancer Research Uk allows you to wear the clothes you want which are also suitable for a public environment as you're always greeting new people entering the shop.
Casual attire and a good attitude to the work enviroment
Be smart, but fashionable - make an effort
What you would wear during your time there, should also be something that shows willing to work and nothing that will put them off
Just your normal clothes as it's volunteer work.
I would wear smart jeans or trousers , a blouse and flat shoes or trainers so that i looked presentable and also so that they could tell that i wanted to volunteer here at cancer research .