LONDON, Jan 31 (Reuters) - British train and bus operator Arriva Plc said on Tuesday it had agreed the sale of its car and commercial vehicle rental division to Northgate Plc for around 129 million pounds ($228 million). Arriva said the proceeds would be applied initially towards repayment of debt and would significantly strengthen its balance sheet at a time when it is pursuing acquisition and franchise opportunities that would strengthen its presence in key markets.