As I understand it the original GWR, The Great Western Railway Company, continued to exist as a legal entity for nearly two more years after the grouping, being formally wound up on 23 December 1949 - as did the LNER and LMS, whereas Southern went into voluntary liquidation on 10 June 1949, having satisfied the requirements of Sections 12, 13 and 24 of the Transport Act 1947.
GWR and its associated trademarks were not taken up until a new company was formed with the same name on 1st December 1994. From looking around, I can see no references to any GWR trademarks being maintained during the BR era