During the Polish campaign the British and French decided on closer co-operation and use of common material. One result of this was the exchange of French 25mm Hotchkiss modèle 1934 against 250 British Boys 13.9mm anti-tank rifles, ordered in March 1040, delivered in the autumn. Hence the BEF had it's 25mm's.
The French had seen that Polish Wz35 7.92mm anti-tank rifles served a purpose against lightly armoured vehicles and was more portable than it's 9mm & 13.2mm machine-guns. The Boys were issued to GRDI (groupe de reconnaissance de division d'infanterie) and air-portable infantry.
For more info on the Boys watch 'Stop That Tank!' 1942
https://www.youtube.com/watch?v=Lkc2yAaU3CY