Thanks everyone for the tips! Now I'm finally done with my December travels and holiday errands.
Turns out, since this time I was travelling with my partner, we had a lot less time to explore the witchy shops. We did go to Watkins, which I found lovely - and what a huge collection of decks! I got a Druid Animal Oracle from there, and I found the last copy of Hexen 2.0... Wow, what a deck! Turns out to be one of my favourites.
There were others I wanted to visit as well, but since my partner also had his wishes, we ended up just walking through the Covent Garden area, where I went to a small shop, Astrology Shop it was literally called. There were also some decks there, but I already owned most of them. Got some incense instead.
We did have a lot of fun with finding a magic shop. There was a "magic dealer" in the Winter Wonderland, but he didn't have any fanning powder - so he recommended a magic shop "right under Charing Cross station". So actually, the station's several entrances are closed right now, and it took us a lot of time to find out how to get to the arcade, and it was like a scene from a movie - there were some homeless people sniffing white powder, very filthy, flickering fluorescent lights... Oh my god, we thought we would get killed. And truly enough, there was a sign in the end of the deserted arcade! MAGIC SHOP!
As we went in, it was a dark room covered in dark red velvet, there was a magician behind the counter teaching card tricks to a 10-year old boy. I did not want to know how that 10 year old boy got there in the first place, I mean really... It seemed like the place wasn't real.
But anyway, the kind magician-cashier told me that fanning powder is no longer carried in magic stores, due to the quality of the cards nowadays. Fanning powder, according to him, was used decades ago, when the card finishing was not of quality high enough to allow for fanning and tricks. When I told him I want to use it for Tarot cards, he didn't really know what to say
but told me I could try some pharmacies for zinc stearate.
I will visit the other recommendations when I visit London next time and keep you posted!