Calls to 03 numbers are charged at exactly the same rate as those to normal 01 or 02 numbers, both from landlines and mobile phones. If you've got a landline package that gives you free calls to 01 or 02 numbers then 03 numbers will also be free. (They must be; it's an Ofcom rule that all phone companies have to abide by). Similarly if you've got 'included minutes' for calls to 01 and 02 numbers from a mobile, you can also use them (without any additional cost) for calls to 03 numbers.
http://consumers.ofcom.org.uk/phone/how-much-does-a-phone-call-really-cost/