The British were winning at first, pushing back no less a general than Rommel. But the French were supposed to advance from the south and catch the Germans between them. The French never appeared. Rommel started to throw 88's at the English. These were originally designed for the Luftwaffe. But they wreaked disaster on British tanks.
From here there was no road for the British but to Dunkirk and defeat --- or so they imagined then.