What to pack for York and London in december

I’d take the raincoat, leave the down jacket. If it rains, you need a longer length cover than a jacket and those packable jackets are not waterproof or even very water resistant ( I have one too and have been soaked through in the rain). Various layers under the rain coat should be enough, but make sure to have woollen gloves, scarf and beanie too, and waterproof shoes/boots. The trouble with North Atlantic island weather is that it’s unpredictable, but rain, wind and cold in any combination are likely. I would definitely expect York to be quite a bit colder than London.

I find the combination of a down puffer and waterproof rain jacket, both hooded, to be very effective. For a trip like this, I'd probably want them to be mid-thigh or nearly knee length. A tip I read on the forum is to wear a ball cap to prop up the hood of your rain jacket. Last winter I found an insulate one at the thrift.
What you don't mention is the bottom half. You'll need comfortable, thick-soled shoes or boots, preferably waterproof and insulated (or able to accommodate wool socks). Baselayer tights (long johns) made of silk if you find the Merino too itchy. Costco usually sells inexpensive synthetic base layers in a two pack. Check at the end of the summer. I would give them a test drive for breathability before committing to them, though.

from the York Weather station archives last December.

in the week 8th-14th December temps never fell below freezing, not even middle of the night, daytimes reached double figures Celsius on most days, 9th even reached something that would have been ok in early April! One day real rain, 3 half days drizzle mostly after dark.

in the week 15th-21st December temps fell below freezing during the night of 16/17th but daytimes always a few degrees Celsius above freezing and touching double figures at times. One day heavy rain, half a day lighter, and three spells of drizzle.

It sounds like you're not planning a lot of outdoor activities, so I wouldn't overthink it too much. The main thing is to have a coat you are warm in, which it sounds like you already have, and shoes that are waterproof. If it's raining, you're probably not going to be spending much time outside anyway (walking around an xmas market in the rain isn't very fun even if you're wearing a lot of waterproof clothing). If it helps, in the winter here I have a long coat (not even a waterproof one), scarf, hat, and gloves, and I generally wear Doc Martin boots. If you need extra layers I really like Uniqlo "heat tech" shirts which I normally wear with a jumper/sweater. For rain I have an umbrella. Good luck with your planning for the trip.

Thanks for all the input.

I have never worn the down jacket in the rain and you are right that might be a disaster. Maybe I should get a longer and bigger raincoat that I can layer more under and leave the down jacket at home.

I have thought about shoes. I have some hiking shoes that do well on cobble stone. I wore them a lot in Greece this summer. I have to spray them to waterproof but that seems to work (we had rain Arcadia in the mountains).

And I also thought about silk or similar long underwear. I don’t want to be sweltering but if didn’t take up much room, it might be worth having just in case.

England has a wealth of second hand clothing charities/shops. In London in particular people tend to be pretty stylish and you can find some excellent stuff. My advice is to pack light and if it's too cold for you, simply buy what you need while you're there.

York is colder than London. As you’re not used to cold I’d definitely recommend thermals. I wear Uniqlo Heattech tops all winter and the leggings are good if it’s very cold or you’re going to be outside a lot.

Down jackets can be really bad in rain. I have a Uniqlo parka that’s water resistant to an extent but no good in heavy rain. If it’s going to rain I wear a Didriksons parka. Make sure you have gloves, a hat and a scarf. Last winter was very mild where I live and I didn’t wear either coat that often. I was usually fine with a lighter jacket, but you’re not used to cold weather and visitors here do seem to find it colder than I’d expect.

Maybe I should get a longer and bigger raincoat that I can layer more
under and leave the down jacket at home.

If you layer, make sure your raincoat is also windproof. Your down jacket as well if you bring it.

Another vote here for Uniqlo Heattech thermal T-shirts and leggings, very light and easy to wear, I live in the tops from November to March, the leggings under jeans when it’s really cold. You can also get similar at Marks & Spencer, but I certainly wouldn’t want to waste my holiday time searching the charity shops for what I need, much easier to pop into Uniqlo in Regent Street or Westfield when you get to London.

I will look into the Uniqlo heattech thermal t shirts and leggings.

Would you just wear them under your usual pants? I generally don't bring jeans to Europe. Should I in December?

My pants tend to be ankle length but I do have some jeans as well as hiking pants. And I do have a pair of black J.J. Jill full length knit pants.

The difference in winter temperatures between York and London will only be a degree or two if that and it is not guaranteed that London will be the warmer of the two.
In my experience of living in both the north and south the weather is very similar but if there is an extreme weather event, hot or cold, it tends to hit harder in the south. The north tends to be wetter but snow is not a definite. Obviously these are all generalisations.

If you are looking at Uniqlo their quilted waistcoats are a good option. They are designed for layering so fit very easily under light jackets to add extra warmth. My sister in law, who lives in Yorkshire, swears by their Heat Tech t-shirts as a thermal layer when she is out and about.
One word of warning I used to really like Uniqlo merino but recently the quality has really dropped off. I bought my mum a couple of jumpers and they were really thin and went to holes in the arms.

Ooh Emma I really don’t agree! I’ve also lived in both and the North East of England is always much chillier than down south. The wind is freezing up there.

Based on the feedback here, I just bought an Eddie Bauer longer rain coat with a hood a size larger than I usually wear. I am hoping will allow for the lightweight down jacket underneath. I got a good deal on it on the Sierra outlet website.

I must admit I kept thinking about what if it is just above freezing and raining. I realized that no combination of clothes in my present wardrobe would be adequate.