As I have pointed out many times, the registrars are unlikely to sit on their hands if there's money to be made. Every domain owner already has an existing relationship with one or other registrar - leaving aside the special case of Nominet tag holders managing their own portfolios - so they are likely to be bombarded with communiques from their registrars hoping to double down (actually, triple down, if the proposed pricing is sustained through to the final implementation) on their domain registration revenue by pimping .uk - easiest money they'll ever make, in many cases!
Even if Nominet only sends the one message (and given the deluge of begging emails I'm receiving from them for each domain we're not renewing, how likely is that really?) it will be far from the only communication on the subject most existing registrants can expect to receive.