Not happy, as a nominet member can we not 'refuse' to accept this change?
You can try, first by making your views known in response to the Terms and Conditions consultation. Be aware that Nominet are trying to remove the bits that require them to set prices on a cost recovery basis, and anything that binds them into consulting before future price rises.
Secondly, you can sign the petitions here
http://www.egm.uk/ and if it gets as far as an EGM, vote for whatever resolutions make sense to you (I don't know what those might be yet)
Thirdly, you can also refuse by proactively approaching your networks (media, MPs, etc.) and spreading the word and explaining just how egregious Nominet's actions are. You will probably need to walk them through the whole thing until they get to the BIG number which is how much Nominet (an entity already in surplus year in year out) stands to make in additional revenue - the price rise per domain is hardly likely to enflame passions.
Fourthly, you can call the Member support team at Nominet and share your views, and get them to bump you up the organisation if you feel they're not going to give you the traction you need.
Beyond the above, there may yet be other mechanisms for affecting change that I've not seen advanced yet. And of course if there were to be something like a class action lawsuit you would be able to join it as a member of the "class", depending on how that was defined.