HMG's line was that the RN would be free to keep the batch 1 Rivers if it could find a way of fitting their running costs within existing planned funding - ie by finding savings elsewhere. With the pressures on manning (and of course funds) I guess it was felt that resourcing full-fat warships was a higher priority. That said, I've heard suggestions that they may look to retain one of the batch 1s, making a total OPV fleet of 6, but have seen nothing confirmed.