waterproof jacket, then some thin layer that is warm when wet - lightweight fleece or merino ls jersey
I have one of those rapha insulated jackets - definitely warm when wet. My jersey will also be (lightweight) merino
you should be fine then. could even leave the insulated jacket and take armwarmers only. depends how fast you intend to be riding!
@gillies started
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waterproof jacket, then some thin layer that is warm when wet - lightweight fleece or merino ls jersey