You can spend them on whatever you like hun, say you have £10 worth of points on your card and you have picked up deodorant/toothpaste/mascara or whatever and it comes to £7.50 for example, just give the lady at the til your card and ask them to take it off your card. They will do that and then you will have £2.50 left on your card plus whatever your points were for that transaction and you don't have to pay for your shopping.
The only thing you can't do is use £10 worth of points towards your shopping if your shopping is say £12. Your card amount has to cover your shopping. Hope that makes sense/helps x