NCVO is currently having a membership drive, and you can find out more about the benefits at https://www.ncvo.org.uk/get-involved/join/.
We are corporate members and find their resources very useful, and they also offer inexpensive training.
Although it won’t apply to most of our insight readers, NCVO are currently offering free membership to organisations with an annual turnover below £30,000. Perhaps you know of smaller charities that you could mention this to?