Why obsolete R-class battleships still mattered

Were the R-class battleships too obsolete to remain useful?

No; although obsolete for the battle fleet, their armor and 15-inch guns made them effective convoy deterrents against surface raiders.

The Revenge-class ships were obsolete for first-line fleet work, but still useful. A single 15-inch-gun battleship could deter Scharnhorst-class raiders from attacking a convoy and impose serious risk even on Bismarck. Its task was to delay, damage, or drive off a raider until faster forces arrived, rather than win a clean duel. This limited mission explains their convoy employment and Britain's willingness to transfer Royal Sovereign to the Soviet Union.