Presumably the posts published on the Nominet forum remain the property of the poster, why not just set up a new forum and republish everything to date, thereby moving the content and forum beyond the control of the board. Data subject access request from each member should do the trick if they refuse to provide a full copy.
Once the paying members have their own forum they can organise, after all nominet doesnt own anything but "*.UK". and ".GB" for example as has already been posted here as an alternative
I have noticed the same behaviour shown by Nominet in other "UK Insistutions", where IMHO the board have set themselves up with benefits and pensions whilst the paying members were looking the other way. This IMHO is what the grab is all about, the nominet board have been allowed to have too much power without oversight and the result is that the paying members have not noticed that they have been systimatically disenfranched over years. The members might be able to fight the grab but it is likely to be both cheaper and more reliable to just take you ball back and start your own game without the leeches.
Biting the hand that feeds IT
