I had an interesting conversation about this yesterday with one of the nice ladies at the Smedley factory shop in Lea Bridge.  It set me thinking.  How about a FOLT plain white cotton t-shirt, a cream merino polo shirt, then a dark green sea island cotton v-neck?  Three slender layers.  A raincoat could go over the top.  You might even get away with a tweed jacket until the weather turns really nasty.