Why do wealthy Americans donate so much to charity and rich Britons so little?
It's a shame that UK law doesn't recognize most of the classes of organizations that in the US would be "501(c)(3)" tax-exempt organizations to whom donations are considered charitable and also tax-deductible.
As I understand it, there is a British equivalent to what Americans call a "non-profit"* -- the "company limited by guarantee." Interaction's parent corporation (of which I am/was a director), UK 2005 Ltd., is such a legal entity. But it's not what the British call a "charity" and it's neither tax-exempt nor do donations to it get you a tax deduction.
Many American conrunning non-profit corporations like SFSFC, MCFI, SCIFI, etc. are both non-profit (or equivalent) corporations in their respective states and also 501(c)(3) tax-exempt organizations. There's a tendency to toss around "non-profit" and "501(c)(3)" and "charity" as if they were interchangeable terms, but they're not -- they all have subtle differences in meaning. In any event, 501(c)(3) organizations do not have to pay income tax on any profit (call it a "surplus" if the technical term for excess of revenues over expenses bothers you), and donations to such organizations are considered charitable donations, and you're allowed to deduct such donations from your income for the purpose of determining your personal income tax.
If UK law allowed the wide variety of organizations that can qualify for US 501(c)(3) status the same benefits as UK charities, I expect the British giving levels would be a bit higher.
*"Non-profit" and "not-for-profit" are functionally equivalent terms. I've never found any substantive legal distinction between them. It appears to me that the term one uses primarily comes down to the state in which you live and the semantic sensibilities of whoever drafted that state's constitution or corporate legislation. California's formal name for incorporated groups of this nature is "California Public Benefit Corporation."