After the execution of Charles I in January 1649, England came under a republican government that provided for distinctive symbols at sea. The parliamentary navy was ordered by the Council of State on 22 February 1649 as follows: "that the ships at sea in service of the State shall onely beare the red Crosse in a white flag". Shortly afterwards, special rank flags for Admirals ans subordinates were approved too.
Sources: Flags of The World (FOTW) website:United Kingdom: Flags of the Interregnum, 1649-1660 | Historical Flags of Our Ancestors website:Flags of Britain and the United Kingdom - Page 1
Sizes: ensign W12 x L18 mm, mast flags W8 x L12 mm, jack W6 x L9 mm, pennant W4 x L48 mm.