Came across
0800buster which is offering calls to 0800, 0808 and 0500 numbers through their landline number.
[Don't know if this was already posted, couldn't find any posts related to this hence posting it.]
They claim that -
Most mobile phone providers in the UK still charge their users for calls to 0800, 0808 and 0500 numbers. 0800Buster lets you call these numbers using your mobile’s inclusive minutes, by providing you with a UK landline number to dial instead.
No registration or set-up is necessary, and the service is completely free*. Just dial 01208 34 0800, enter the full free-phone number you want to call at the prompt, then press #—it’s that simple!