You can quickly cool down when it's raining out, in particular when the rain is horizontal and it's 4°C outside. You say there's no shelter anywhere near. Then walk as fast as you can to somewhere not near. I infer yours must be a day trip, for surely you're not out on a multi-night backpacking trip in a cold & wet climate without any raingear or night shelter.
Locate the closest shelter, be it a bus stop, a pub, your car, or even a house if you trust strangers and they trust you. Even if they don't invite you in, you might hide under a ledge. Walk to such shelter as fast as you reasonably can, be it one, two, or three hours march. The wet-cold spiral is real and bad for you.
P.S. It amazes me how often I see people in England out in the rain without proper rain protection. This is England, it rains. I don't recall seeing the same in Netherlands, Sweden, or Norway. In the USA too, right on the peak of Mt. Marcy, I remember seeing hikers without any rain gear caught out in heavy rain. A sure way to catch a cold or worse.