Can't help you with the alleyway shop Steve, the bag incident sounds like the sort of thing that happened at The Popular Book Centres, where you returned books and got 50% off your next peurchse.
I don't think they dealt in memorabilia though.. Books and comic Exchange used to be in Cambridge Circus area, but may have moved to Glasshouse street by 1989, I know I bought a Yellow Submarine linen backed American film poster in there at around that time.
This wasn't the signing your brother and you attended,as A,D, failed to show and Mark Hamill stepped in...
Here's a link to FP's archive , you may be able to find that signing date.
https://forbiddenplanet.com/plu/posts/?tag=archive
Clicking on the details takes you to photos of the event , or stuff like this Cinema ad from 1979, shown at the London performances of Alien.