As far as I know the AC voltage rating of filter capacitors refers to the RMS voltage of the AC supply, not the peak voltage. In bygone days of 220 V, X and Y caps were usually rated for 250 V and those designed for the international market (including the UK) were usually 275 V ones. To this day, many electronics stores sell 250 V filter caps, presumably new old stock from the 1970s and 1980s. Before the X and Y designations were invented, filter capacitors had either dual DC/AC ratings, usually not exceeding 250 V AC either, or only DC ratings, usually at least 600 V DC.
