Many British units began their retreat from the Dyle position towards France around 14 May, as Belgian forces had begun collapsing on their left flank in the couple of days prior. The 15th/19th Hussars, on the BEF's extreme left around Louvain, noted that by 12 May Belgian troops were joining with the refugees streaming back through their lines, and that the signal for retreat was given two days later due to the disintegration of the Belgian unit directly next to them. The general retreat order from General Gort came two days after that, on 16 May, with orders for the entire BEF to begin falling back from their advanced positions, along with the 1st Army Group they were attached to.

Source - Bovington, The Tank Museum Archive (BOV): ‘War Diary of 15th/19th Hussars, RAC’