Hobgoblin is your best bet. They don't just sell guitars and squeezeboxes.

And at 24 Rathbone Place, they are just off Oxford Street near Tottenham Court Road, and there's a decent pub next door, and a coffee shop. (There's a map on that link.)

Free Reed have just about everything, and it'd be worth giving them a ring. They haven't got a shop as such that I know of, but that needn't mean they couldn't help you.