Thanks for the replies. I thought Nominet were the basic 'cheap as chips' registrars - got that wrong!
Nominet are the not-for-profit company tasked with managing the .uk namespace. As such they are the people who hold the zone records to make things work. This needs to be centrally coordinated for obvious reasons.
Now you can do things through them if you should want or need to, but they are not interested in contact with individual end users. Hence the prices!
The only time you might typically need to do something direct with them is if you were trying to renew or transfer the admin tag to a new agent AND your existing one was bust, dead or not replying etc..