The only place I'm aware of that sells Queen official merchandise is at the Dominion Theatre. They have mostly We Will Rock You stuff, but I got a black shirt with the Queen crest on from the merchandise stand inside the theatre, and I think it may be open during the day.
You can get to the Dominion Theatre very easily using the Underground. I just walked out of the Tottenham Court Road station and was right across the street from the theatre. You can't miss it, the huge "Freddie" statue is right there on the theatre marquee.
Here's a map of the London Underground. Click on the box when it appears in the lower right corner and the map gets bigger and easier to read. http://www.oxfordtube.com/assets/london/underground_map.jpg
Have a terrific time in London!
I saved Spike's life in 'Nam.