Outerwear for London/Barcelona/Italy in December/January

Not sure about Barcelona & Italy, but for London I'd suggest a midweight WATERPROOF coat and some layers underneath. A lightweight fleece sweater can be a huge benefit and they're easier to pack than heavy winter woollies. Bring a hat & scarf. London isn't often below freezing in December but can be wet & a bit breezy.

You are always better off using layers. That way you can more easily adjusted to the temps. In summer I carry a nice, wind breaker, waterproofing shell. In the winter I add a medium weight down vest couple that with a medium weight wool sweater, maybe a turtleneck, with long underwear, I am good to about 0F. I will use wool blend pants with long underwear and that covers pretty well. Since I am not getting sweaty from the summer sun, we actually take few clothes because they do not get as dirty as quick. For example - instead of three or four short sleeve shirts will take two long sleeve and one short sleeve. Since wool pants are a little heavier we only take two pair instead of three pair of lighter weight pants. We pack lighter for cold weather than we do for hot.

Plan for all kinds of weather and you'll be covered. You just know it'll be cold and wet in London, but depending where you go in Italy you might find the same. We froze our asses off in Milan one fall - we called downstairs at the hotel to ask for heat and they said it hadn't been turned on yet for the season!

Yes, follow Jane's advice to use mid weight and layer beneath. We spent most of last December and half of this past January in Amsterdam. Hubby took a waterproof puffer jacket and I took a "water proofed" water resistant jacket with built in fleece. It performed admirably when it rained and sleeted/snowed. I used Land's End thermaskins top and bottoms almost every day under my shirt and jeans, and on really cold days I began my layers with a Cuddlduds fleece top and bottom as my first layer. I took one set of each. Cuddlduds are sold at Walmart. You can also use them as pj's if you find you don't need them for day wear. Merino wool socks kept our feet dry and warm. I have had good luck with WigWam merino socks. I'm going on two years of fairly regular wear with my first three pairs (except summer when I rarely wear shoes), and they are still going strong. I think hubby bought light or mid weight Magellan merino socks at Academy Sports.
Hubby mostly wore merino wool long sleeve shirts as an undershirt on about half of our days and only felt he needed his Magellan long underwear pants twice.
If you are not a regular shopper at Land's End you can sign up and they will send you 30% of reg price coupons almost weekly. I just redeemed a 50% off coupon and got another Thermaskin shirt for my trip this fall.
Have a wonderful trip!

I find the following items will take me though any type of weather:

• waterPROOF shell
• puff jacket
• merino cardigan
• ultralight silk thermal shirt/leggings
• wool socks
• hat
• good waterproof gloves or mittens
• light scarf
• waterproof shoes

If I'm outside a lot I'll also bring:

• waterproof pants (Montbell Versalite)
• another pair of long john bottoms
• base layer top ( I prefer t zip neck for thermal control)

This combo will take you from 60 F to -20 F easy. I say this from experience. I was able to take most of this in a 16 L bag so that should tell you how compact and light it is.

I would just say layers, layers, layers.

A waterproof shell, fleece jacket, normal shirts, then some type of thermal undershirt. Mix and match depending on the weather, a thermal undershirt can serve as a top by itself, so count that as a change of shirt. I would also take thermal long pants to wear underneath as well, UnderArmor makes some nice things. With the four upper layers mentioned, I am not sure if you would need a dedicated winter coat, but if you think you might want one, I would look at some of the ultra-lightweight fiber-fill jackets, my wife has one from Patagonia that can stuff into a small bag smaller than a football.

If this somewhat helps answer your question (although I can't speak to Barcelona), I took the RS Paris tour in late December a few years ago, and wore essentially what I was wearing back home: a wool sweater, jeans, a quilted jacket (think LL Bean quilted riding jacket) and had a scarf, gloves and toboggan with me. (Toboggan can mean knitted hat where I grew up.) I wore either short leather walking shoes or running shoes. It was all good.

Just keep checking the forecasts as you get closer to trip time because as we all know, each winter differs.

My standard advice is, no jeans. They're heavy and bulky to pack and wear and heaven forbid they get wet (which they will when it rains).

Italy spans a range of climatic zones from the alps in the north to Naples and further south with Mediterranean weather. Common need would be a rain jacket which also blocks wind. Then a variety of insulation layers from a light fleece to something warmer.
https://www.rei.com/learn/expert-advice/layering-basics.html

Like thermal underwear, fleece tops are available in 3 weights:

Lightweight for aerobic activity or mild climates.

Midweight for moderate activity or climates.

Expedition-weight for low activity or cold climates.

We lived in Rome for several years and traveled throughout Europe in the winter. I found in Rome I needed a °puffy coat° as so many women wear there. I bought a long one, well below the knees, as we were out-and-about all the time, walking and using public trans. I used the very same coat in Paris and London in winter.

On the days it was warmer in Rome, I might get by with a long sleeved shirt layered with a sweater plus a light jacket. In London, I was always happy to have my puffy coat!

It also depends on what you are used to. If you are from Minnesota and used to bone-chilling cold, you are not going to have a problem with layers and a mid-weight, rain resistant coat. If you are from Louisiana, get the puffy coat.