I would say in this case tell whoever stops you that you are taking their car registration number and calling the police. Make sure that your phone has the police number programmed into it which is 155.
I really try to take notice of car registration numbers and makes of cars. It is the first thing police ask for.
I also have two purses in my bag, one for smaller amounts of money for meals, fares and things like that and one for my credit cards and larger amounts of money and coins. If it is really hot and you get flustered it is so easy to get short changed.