Cheapest way to get an 0800 or 0845 number

A friend (accountant) is about to run an advertising campaign in a variety of local media.

He works from home or from client's premises, and wants to set up either an 0800 or 0845 number (that he'll forward to his home or mobile) for the purpose of monitoring the effectiveness of the campaign, but also so people aren't put off calling for more info due to the likely costs involved.

He's asked my advice, but before I get to work searching google, does anyone know anything about this and could help me out? Pros and cons? Costs involved? Best option to choose?

Thanks in advance!