Well done for having a go.
A few years ago my wife did this as an extra for us. she charged £9 per hour at the time.
It may be better to do it by weight.
Do your own ironing and time yourself. Weigh at the start and work out how much you should charge per kg. Have a min charge. In time you will be able to work out how much it will be by the size of the bag.
Go to your local insurance brokers and get Public Liability and register with the National insurance. If you expect to make less that £5000 you are exempt NI. You wont pay tax either. You will need to inform the Tax Credits but remember the system of Tax Credits rewards work so if you earn £50 per week you wont loose £50 per week.
As for how much you should expect per hour anything from £9-£15 but remember if you charge by the weight the customer wont know your £ph and they dont need to either.
Keep a note of the weight for each customer and of all payments.
Hope this helps.
Stuart