Freephone calls are meant to be, er, free - not that you would know that if you dialled from a mobile where it can cost up to 33p/minute. And while the regulator Ofcom has published plans to change this from 2014, there are ways to already call 0800s for free from your device.
The easy way is via 0800buster.com. It will give you a normal local phone number to call. Then, anytime you need to call an 0800 number, just dial that number first to access its system, then the freephone number and then press # to start the call. Those with smartphones can download apps which do much the same thing, such as the 0800 Wizard.
If you don't have inclusive minutes you will be charged standard local rates for these calls – though in some cases it is cheaper than the cost of calling 0800 numbers from mobiles anyway. See www.moneysavingexpert.com/0800buster.