If you don't already have a turtleneck, go buy one. Now. Hell, even if you own a few, you should consider a one more -- maybe in super thin merino wool or cashmere (it'll take up less room in your suitcase).
It's a Saturday afternoon and, having told me that my wardrobe is a little "tired", my girlfriend decides to take me shopping in Shoreditch. We go to a store called A.P.C. It's French, which is apparently justification for charging £75 for a t-shirt.