if your plan in London is to go shopping for Christmas items, you're in luck. The season is in full swing. The Selfridges basement has a very large and very busy section for Christmas goodies. My wife asked the weary salesgirl if it was like this every day and she basically said yes on the weekends, especially when it's rainy. We also popped into Liberty which is a really cool building, I think I read they have a department as well and I'm sure by now Hannah Ricketts has done her annual vlog about Harrods. F&M was of course seasonal and of course batsh*t crazy.
Outside on Oxford Street is a good place to do comparison shopping for souvenirs, also a good place to pick up any plug adapters you might have missed. They also had lots of luggage for sale but it didn't seem especially cheap, my general rule in Europe is that outside major train stations you can get great deals on suitcases for all the lovely junk you're bought on your trip but that doesn't seem to work so well in London.