I have taken the time to review the relevant legislation and it seems you would be unable to take any action, whether it be to have the post taken down or for damages because you are a charity.
Defamation Act 2005 s. 9(2)(a) -
A corporation is an excluded corporation if —
(a) the objects for which it is formed do not include obtaining financial gain for its members or corporators
An excluded corporation is a corporation that cannot take action for "defamation".
Since you're a charity it would follow the corporation was not formed with the objective of obtaining financial gain.
DM