As you may be aware, calls to supposedly free 0800 numbers from your mobile telephone are usually charged at national rate and are excluded from bundles of free inclusive minutes. However there is a solution!
Store this magic number in your mobile handset: 020 0222 0700 (also 020 0222 0900)
This number is interpreted as a standard 020 London number by most mobile operators and once connected it allows you to onward-dial any 0800 number at no additional charge.
Also don’t forget Say No To 0870 which lists alternative geographic numbers for most UK businesses.