The cost of becoming a Nominet member is pretty low, if I remember correctly its around ã500.
In regards to enom under cutting for .uk domain names, in my personal opinion I believe this would be impossible for enom to accomplish due to one reason. Exchange rates.
As seen as enom only operates in united states dollars, it would be impossible for it to under-cut rival nominet tag holders as it would be at the mercy of exchange rates between the dollar and the pound. If exchange rates changed to not in their favour, they could end up selling at a loss.
And the second most important point is that Enom are an american entity, so why would they try marketing .uk domains on price as at the end of the say, british people prefer to pay in pounds sterling.