Nominet just got back to me
"Each registrar who has signed up to the RoR release has been allocated an initial number of registration attempts, based purely on the number of domains they told us they intended to register during the release period. That means registrars with larger numbers of attempts are not all big registrars, but those that intend registering high volumes of domains.
To balance this, and ensure we are being fair to end users, we want a level of confidence that each registrar has the funds in place to cover the costs of this number of registrations. If you would like to register a larger number of domains than you initially advised us of please email me the number of domains you intend to register, and add funds to your account to cover the value of these registrations. Unallocated funds can be added to your account in the ‘Billing Summary’ section of your account.
The final allocation for each registrar will depend on how many total registrations are requested and the number of accounts signed up. The more registrations and accounts then the fewer attempts will be given to each registrar as we can't exceed a certain number as we need to protect our systems. Once we are in a position to see this (i.e. by midday on the 21st June) we can then finalise the allocation of attempts per registrar and will communicate this to each contact w/c24th June."