The reality is that if you Are not able to fund the action then it is not available to you.
This is a defamation and you do have that action open to you. It is a very expensive action though.
You could always get a solicitor or a barrister under public access to write to them a cease-and-desist letter and often that works. That would cost about £100 plus VAT. It has no legal status of itself but it is often very effective.
Failing that, your only option is to sue
I would suggest that you consider an action in harassment which is cheaper but if they are complaining about an organisation rather than individual that is not really easy to do.
Can I clarify anything for you?