Thanks for your messages everyone; I'm happy to report that he is home, safe and sound. After 4 weeks, he was found near Victoria station, about 2.5 miles away from my house. I have no idea how he got there, including crossing the Thames somehow. Thankfully he is completely unharmed and even seems to have kept himself relatively clean and fed while he was roaming around.
Good luck to everyone who is still searching for their lost one; hopefully my saga will cheer you up and reassure you that cats are super resilient, and often come home even after a long time being away. Oh yes, and microchips are the best invention since sliced bread and beer.