We use two Mountain Equipment bags ( Left and Right Handed zips ) Not sure if they still sell them but might be worth looking. Not down but still compress very well, fairly pricey but very good
Bags used to be marketed by “season”. If that’s still the case, go for 5 season (though it’ll be too hot for summer then). Remember the zip will be the weakest area for keeping out the cold.
And you can do other simple things: it does feel strange to wear a woolly hat in bed, but that’s where you lose heat. I use a hot water bottle at home too, never mind camping.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.