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Can't say I've ever had it that warm and raining in Scotland. But in that temperature id just have a merino layer and a rain jacket. Then accept the merino might get damp from sweat and swap it later for a dry one when suitable, or it'll dry itself. You can undo zips but ultimately you'll sweat. So just accept it and operate with it.
The alternative is to alter your effort and march it with suitable clothing levels so you don't sweat. Ultimately I don't like to ride that slow, but it's a smart move, both if it's raining but also if it's hot and sunny too.
Got some bikepacking in Scotland coming up. Need a new rain jacket and the Zoa rain shell looks like it might work well. Especially keen on the hood. How does it handle warmer wet days? Is it a sweat fest? By warmer I mean 12-18C, not actually warm.
I tried the All Road rain jacket on in Condor the other week and even up a size from my usual Rapha size it was a bit more 'race fit' than I was looking for.