As much as I love the Metro lifestyle, setting the majority of the RP underground would severely limit what kinds of animals or areas the characters would find. On the surface there are tons of locations we could use; I reckon the Isle of Dogs would be a great place for a community living in the wrecks of the old towers.
Interesting point though. There wouldn't be too many overgrown animals or mutants around in Britain due to FEV being an American product. We could find a way around that (prewar bioterrorism, black ops stealing phials etc), or else come up with a European analogue. If we want a mutant presence anyway.