Ted Baker London polo shirt Euro size 5 tag, US size small

A nearly flawless Ted Baker polo shirt. Great quality. Fine navy blue and white stripes. While online sources say a TB size 5 is a 42, this shirt measures 20" pit to pit. The sleeves are 8" from seam to cuff; back is 26" from bottom of the collar to the bottom of the shirt. Shoulders are 16 1/2" seam to seam. 100% cotton and made in Turkey. I would keep this but I really need a medium.