There are a few mag shops in soho 🙄 that do all the fashion/art titles as well as other ‘special interest’ publications, think there’s one on wardour st not far from the toucan pub and another on old Compton st before you get to the posh drinks shop, there is also the stall at the back of the theatre near the crossing outside Liberty no idea if they do your magazine.
Is there still a borders books at Angel? They used to do all the mags there and I think it was still open after the charing cross branch closed.
If you phone the publisher they should be able to tell you where it’s stocked.