LONDON, June 19 (Reuters) - British supermarket group Morrisons (MRW.L) has rejected a proposed cash offer of 230 pence a share from private equity firm Clayton, Dubilier & Rice (CD&R), it said on Saturday. Morrisons said the "unsolicited highly conditional non-binding proposal", which was received on June 14, "significantly undervalued Morrisons and its future prospects."