Liberty: The high-end and stylish Christmas Shoppe is a must see and you won't want to leave without one of their trademark purple ornaments especially if you are visiting as a tourist. Also, don't miss the Wild at Heart pop up shop at the entrance that has gorgeous wreaths and holiday foliage.
Paper Chase: The name states the obvious and they do carry all of your paper needs. Their multi-level flagship store on Tottenham Court Road is worth a visit for their funky Christmas decorations and a wide range of gifts.
Heals: A one-stop shop for holiday decorations, gifts, and a wide selection of gourmet holiday items.
Harrods: Over the top and overpriced, but with an early opening in July, their Christmas shop is a great way to get into the holiday season.
John Lewis: A department store similar to Macy's that you can find almost anything imaginable inside of.
Selfridges: A large department store that has a number of fun pop up shops this season including Pedlars and Anthropologie. Don't miss their white room filled with decorations and gifts solely in shades of white.
DZD: For when you want your Christmas display to be larger than life you need to stop by this store in central London specializing in large scale holiday decor for retail shops.
Fortnum and Mason: The ultimate shop for gourmet food gifts and holiday gift baskets as well as a fun selection of holiday decor.
Of course no home would be complete at Christmas without a fresh cut Christmas tree and there are a ton of great options in London.
Columbia Road Flower Market and Borough Market have fresh trees from local farmers and you will see tons of locals lugging their trees over their shoulder home on weekend mornings.
B & Q and Homebase are the Home Depot alternatives in the UK
The Camden Garden Centre offers a large selection of trees and delivery is available.
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I admittedly haven't been there in years (sadly), but The Christmas Shop in Hay's Galleria, near London Bridge has a lot of really nice Christmas items for sale.
Nice, I'm spending 3 weeks in London this Christmas and my boyfriend (who lives there) loves Christmas and the decorations, but is just totally oblivious to the notion that you have to actually put them up, they don't just appear. I will have to check these out so I can deck his halls for him.
The English are geniuses at paper decorations for the holidays. I treasure the ones I bought when I lived in London 30 years ago for a year. Stock up if you like them, because you really won't see anything just like them over here in the USA
Great list but I'd add V. V. Rouleaux for ribbons, a don't-miss-stop on my visits there, great for both supplies and inspiration.
You should try Spitalfields market (near Liverpool Street Station) for unusual handmade stuff - local designers and design students have stalls there. There are nice independent shops in that area too. Oxfam charity shops sometimes have lovely Fairtrade handmade decorations (and plenty of Fairtrade seasonal chocolates and other foods). Visit the Geffrye Museum to see living rooms from different eras all decorated for Christmas - it's near Columbia Rd and Spitalfields and entry is free.