The problem is there is effectively zero public pressure for this.
The Freedom of Information Act was only introduced after decades of campaigning by individuals and organisations philosophically committed and institutionally designed to keeping check on the state (aside: check out these fascinating first issues of the 'Secrets' magazine from 1984 onwards).
As long as there aren't organisations that represent charity-givers the government won't legislate in this area. What we need is an association that derives its legitimacy from a broad base of support among regular charity-givers. It would be dedicated towards improving the effectiveness of charities and advising members on where to put their money.
(New Philanthropy Capital is an interesting organisation but seem to be more of a consultancy for high-value donors than a broad association for all charity-givers).
In my opinion only a body like the one I've described could effectively mount a campaign for the legislation that Martin rightly calls for.