what to wear

I would take something warm and lightweight. I tend to prefer nothing longer than hip or at most knee length. Although London will be cold and damp, all the buildings, shops, the tube, the pubs etc will be (to me) over heated so you may be constantly taking a coat off, if not more layers.

I personally would find a fleece lined coat way too warm. And you are unlikely to get a downpour.

Nice jeans are totally acceptable, in my opinion anyway. I wear jeans constantly at home, and do same on holiday.

Don't overdress. London rarely gets bitterly cold. Any kind of a parka is overkill, too bulky and you'll quickly become soaked from the inside out with sweat in London weather. I usually find either a warm fleece or an overcoat is more than adequate for most winter conditions in northwest Europe (minus the Alps and Scandinavia, of course). If the temperature drops to colder than normal conditions, I just layer more underneath.

Several thin layers is the usual advice and usually more efficient in keeping you warm. If it does get cold a hat, gloves and scarf will make all the difference and can be picked up very cheaply while you are here.

Here's what I pack:

http://www.polyvore.com/mens_travel_pack_list/set?id=1440268

It's made with shoulder season North of the Alps in mind. You might go a little warmer than this (slightly heavier sweater and fleece, add a knit cap), but it's based on layers so should cover a good range of temperatures.

I made this because it seemed a good alternative to writing out a packing list and you can click on an item for more details.

The main thing you will want from your parka is wind and rain protection; wear layers beneath for warmth. Look for a waterproof parka rather than water resistant (surprisingly hard to find in US department stores). A hood is a definite plus as is a length that keeps your bottom dry. You will be glad to have water proof shoes. All other items should dry quickly and keep you warm even when damp or wet (cotton doesn't do either).

I was in London in mid March.. I found it freezing( it even snowed but didn't stick) and the wind cuts you like a knife. The whole week I was there I could not believe how cold it was ( it was so damp thats why).
I had a mid thigh length wool coat( a Pea coat I guess you would call it). Coat was ok,, but, I wished it was longer( to knees) and I wished I had brought hat and gloves,,( I did end up buying some).

I think ones tolerance to weather extremes is highly individule though,, what I considered horrid another may have thought was not too bad. I would wait till before I left and check weather report too. And bring an umbrella.

Useful items for "blending in" and staying warm: black jeans, a cashmere sweater (weightless but very warm), very comfortable dark walking shoes, a black raincoat that is roomy enough to accommodate 2-3 inner layers as needed. Add a long colorful scarf looped European style and you will fit into London perfectly. (Of course everyone knows you are American as soon as you talk, but most of the English seem to like us.) Umbrellas and warm gloves and scarves are available in much greater choice in London than they are in California.

I should add, even though I don't think it's necessary to try to hide your status as a tourist, doing so is easier in the winter. You can hide the usual accouterments of traveling (cameras, guidebooks, maps, moneybelt, etc- these are usually more of a give-away than clothes) more easily under bulky winter clothing.

We once went to England in December and found it freezing! We both wore our usual winter coats (as you can see we are Canadian). We didn't need boots though -- hiking shoes were sufficient. A lot of the museums had coat checks so we were comfortable when indoors. Jeans are more than fine. If you have room in your luggage take a lighter windbreaker type of jacket in case they are having a warmer spell.

Hello Renee! My husband I have been to London a number of times (including winter) and I would go with with a less cumbersome coat - perhaps a raincoat with a zip in/zip out lining. I have one from Land's End and it works great - I believe Travelsmith also makes a nice one with an optional liner. Layering is indeed the key - especially if you are not checking luggage (which I highly recommend!).
Nice jeans are perfectly acceptable for day wear - the only issue is that if they get wet they do take quite a while to dry. Comfortable shoes are a must !! Hat, scarf and gloves as the previous poster said will also be very useful, if you can't find anything at home, no worries - you can find them everywhere in London. I would also suggest a pair of long underwear, top and bottom - either silk or synthetic. They take up virtually no space in your luggage and really do keep you warm. My feet get very cold and so I brought a couple of pair of sock liners which helped quite a bit.
London is one of my favorite cities - I hope you have a wonderful trip!!

I know you said November and December, but I met someone that went to London and Paris for New Years Eve, and said it was so cold there, she would never travel there in the winter again! But she's from Texas, and is used to the heat! What is cold for her, may not be cold to you. But still, bring the long underwear just in case!
Nancy, I have a Lands End coat too and just love it!