I have a 37" chest and wear the XS made in England Fred Perry shirt. It costs Â£55 in the offical shop in Covent Garden. Its a tight fit, but I've tried a Small and it seemed too long and not so fitted on me. I am 5'11" and pretty slim but not overly skinny. On a 40 degree wash they do shrink a bit, and don't tumble dry them either! They had XS on the website a few weeks ago, but they sold out and haven't reappeared. Small in the HK or Portugal or CDG are like a Medium in the Made in England.
One other thing, I ordered a Small of their official website ages ago, and it fits perfectly, however, its actually smaller than my XS, so I assume they just sewed the wrong tag in or something.