Christmas and NYE in Europe for the first time

We loved Christmas in London in 2014. Dec 23 Christmas service at Westminster Abbey. Shopping streets have Christmas lights. Harry Potter Studios at Christmas(reserved far in advance). We are also not fans of cold weather, which Is why I doubt we'll ever go to Christmas Markets.

In a word - Vienna !

Germany/Germanic countries as well as countries with German influence are good for Christmas markets.

Estonia can also be a good option because of the large orthodox population (i.e. the country doesn't shut down over Christmas as many celebrate Christmas in January).

The weather in Spain is some of the best in Europe that time of year.
You should watch Rick Steves videos on Spain to get an idea if you are interested in what Spain has to offer :
https://www.ricksteves.com/watch-read-listen/video/tv-show
We had 70 degrees and sunny in Cadiz on the Atlantic Ocean on New Years Day 2019. There is a three kings day (Epiphany) parade in most major cities in Spain on January 6.

I think that Spain might fit your bill! In Spain our holiday season extends into January, with the main celebration being Dia de los Reyes Magos (three kings day) on January 6th. So our holiday festivities extends past Christmas day and through New Years. Plus, Nov through Jan should be decent weather in most of central and southern Spain.

Also, be sure not to miss our Christmas markets in Spain! Our Christmas market tradition is a little know part of our culture (It's not just the Germans lol). Since we mainly celebrate the Three Kings Day (Jan 6), you will find most of our Christmas Markets are still up into early Jan.

You can consider Sevilla and Madrid or both!

Here's some more info on our amazing Christmas markets in Spain.
https://theculturetrip.com/europe/spain/articles/12-magical-christmas-markets-to-visit-in-spain/

We spent Christmas in London with our family a few years ago. It took a lot of advance planning helped by friends who live there. Many restaurants will be closed and you need to book reservations at those that will be open.
We were able to book tickets for the Christmas Eve service at Westminster Abbey, beautiful. Tickets are released in batches in October and November.
Nothing much is open Christmas Day.

Nobody does Christmas like the Germanic countries, but being a tourist from Christmas Eve to New Years can be challenging, as most people go home to be with their families and some businesses even close until after Jan 6th. And it's cold, although usually not insanely cold (that comes in January). German NYE is also a sight to behold, although some find it to be a bit too much. I think it's worth it once in your lifetime, though.

There's no guarantee of good weather anywhere in Europe at that time except perhaps the Canary Islands but southern Spain would probably be best for that.

not a huge fan of cold weather

Nordic saying:

There is no cold or bad weather, just wrong clothing.

If you want to experience white Christmas you have to look for snow-guarantee locations whick means higher altitudes in Austria and Switzerland or some parts of Norway.

German Christmas Markets are very individual; one can be more religious, another more on selling and creating handcraft pieces, or even only full partyish without any reference to Christmas except food and some lights. In the North of Germany you will find some markets leaning more to St. Lucia, so light oriented. Also good to know that not all markets are there the full 4 weeks before Christmas week. Some open only for one weekend for example - some of them are very nice.

Spain would be my first choice for the light, temperature, and wonderful festivities that go on and on.

You may want to join some organized group for the holiday week so you celebrate these important meals with others. A vacation village with meals and activities in common in Andalusia, or a holiday Mediterranean cruise? You may have to look at British companies for these due to language or companies from other countries whose language you speak.

Does anyone have any thoughts on the south of France/French Riviera for about 2 1/2 months in the fall including Christmas and New Years?

Thanks