With Give As You Earn – the UK's largest payroll giving scheme – you can donate directly from your pre–tax salary, so money that would normally go to the taxman goes to charity instead. This means that if you are a basic–rate taxpayer and pledge £10 a month, it will cost you only £7.80.
Your employer needs to be registered with the Payroll Giving agency. They may already be registered - you can find out from your Manager or HR department. More information is available here.