Why not a conventional carrier and take it on bus/tube? Or even a cargo bike (on some padding). Most backpacks I've seen seem smaller and less comfortable for the cat.
I find carrying out cat carrier 500m to the vets awkward, because of the one sidedness and distance out from your side. Wouldn't want to take the wee beastie too much further like that.
Why not a conventional carrier and take it on bus/tube? Or even a cargo bike (on some padding). Most backpacks I've seen seem smaller and less comfortable for the cat.