A Grant of Probate or a Grant of Letters of Administration is not always required. If the person who has died has less than £5000 in total or owned everything jointly with someone else a Grant of Representation is unlikely to be required.
Some banks are prepared to close accounts up to £25,000 without requiring a Grant of Representation but usually a Grant is required when the person who has died has a house, more than £5000, stocks/shares.