Poundland gilets are extremely lightweight, thin and packable, and claim to conform to recent French (not PBP) legal requirement"". And they cost, er, £1.
"" as such, they are more 'hi-viz' than 'reflective'. I can vouch that in wet conditions, if worn over a dark garment, they are actually almost invisible. But legal. PBP requirement will presumably drop into line with the new French legislation - otherwise you might need to carry 2 gilets, a hi-viz one for day (French law) and a reflective one for night (PBP).