Yet he agreed to speak at the Charity Communications conference last week because he knows his name is a draw.
Campbell says he has used his 'brand' to help raise £2m for Leukaemia Research by taking part in races and triathlons. Cathy Gilman, chief executive of Leukaemia Research, estimates that he has been responsible for the charity raising five times that amount.
He chose leukaemia as a cause because his best friend and his best friend's daughter died of the disease. But he confesses he can't actually remember why he went for Leukaemia Research rather than any other leukaemia charity.
He has stuck with Leukaemia Research for the past four years, however. He is regularly asked by other charities to help them, but he nearly always says no. He'll do the occasional talk or appearance, but will not fundraise for other charities because he feels all concerned would lose out. His brand would be diminished and nobody would know which charity was his passion.
Clearly, having celebrity supporters can reap benefits - but only if charities know how to get the most out of them. The media officer of a less well-known children's charity recently spoke of her disappointment at the media launch of a new information pack for schools.
The pack was designed to help schools welcome asylum-seeking and refugee children. The charity persuaded a local football star, who himself had been a refugee, to launch the pack. But it got very little coverage. The media won't cover an event just because a star is present.
A celebrity can only ever be the icing on the cake and events have to be interesting enough in their own right for the media to cover them. With the pack for asylum-seeking and refugee children, there was no journalistic story to report on.
So many events are now fronted by celebrities that the media can pick and choose. Their coverage will be dictated by the story itself and the profile of the celebrity.
Only the presence of a major but underexposed celebrity will more or less guarantee coverage. For example, the National Association for People Abused in Childhood, which is a small charity, persuaded Jerry Hall to launch its new helpline.
Hall used the occasion to reveal that she had spent her childhood living in fear of her violent father. This got a charity with no PR officer and few resources widespread national coverage. So the right celebrity with the right story can put a small charity on the front pages.
- Penelope Gibbs is director of online contacts book askCHARITY