There are dial-through services out there that offer a geographic rate number which, when called, allow onward connection to a freephone (0800/0808/0500) number.

Examples are 020 0222 0900 and 0800buster.

Once the call answers it will request you input the destination 0800 number. Do so, but don't press the phone-off-the-hook ("green") button again or else it will start a second call which is what you don't want. You simply enter the number and then it will connect you. Obviously charging begins when the service answers and requests you input the destination number.

The only mobile service I know of that offers genuinely free calls to freephone numbers is giffgaff which uses the O2 network. As part of the reform of non-geographic numbers, freephone numbers will become free to call from mobile phones next year.