The business is private and has competition. We have other competing registries with one, Verisign's .com, providing significant competition in the marketplace. Verisign will consider Nominet as a competitor within domain names.
But .com has been around since prehistoric times, in web terms. It's not magically "more of a competitor" now than it has been in the past. In fact, since companies all over the world fight over England-language .com domains (driving prices higher) it's probably less of a competitor than it might once have been.
I was familiar with the general theme of the Registrant T&Cs once I was reminded of the existing content by Nominet (I don't review then very often). Registrants don't generally purchase services from Nominet very often. A small number purchase domain names directly at £80 + VAT. Instead most purchase domain names from Registrars who set their retail prices accordingly. Those are the prices most Registrants pay, prices not subject to the T&Cs because they are not set by Nominet, and this offers competition, something Nominet does not influence.
Of course they do! The wholesale price affects the retail price, it's impossible that it wouldn't. Registrants don't pull their registration price out of thin air. The
starting point for their calculation is the wholesale price (though they add variable amounts of markup, granted) If Nominet puts the wholesale price up, the retail price will trend up too. Just like it would in any other industry. That's pretty much commerce 101, very very basic stuff!
Now you may be right that we can't predict the EXACT price rise for a specific Registrar - but that's a very, very different (and much less important) statement than suggesting that there wouldn't be a price rise.
The fact that you don't seem to understand this suggests two equally disturbing possibilities:
A) You don't understand the fundamentals of how companies set their prices
or
B) You understand perfectly well, but are deliberately pretending to misunderstand to try and drive the discussion off course
Regardless of whether it's A) or B), you should realise that your contribution is not helping matters.