First, the Heathrow Express is amazing. For $33, you're in the heart of London in 16 minutes, directly from your terminal. You can buy tickets through their mobile app or at kiosks in the airport. It's perfect.
I walked all over London, mostly by accident. I stayed in Tower Hill, which is a lovely neighborhood one mile from my office. I spent at least an hour every day lost because my phone did not have mobile data while abroad. It was only on my very last night in London I took a Jack the Ripper walking tour and realized I spent the balance of the week traversing the locales haunted by England's most infamous unsolved serial killer. Lovely.
I did manage to get fish and chips at Poppies, which my coworkers assured me was THE BEST place to get it. I got the cod version and it was awesome. The fries were kind of meh, I like my crispier, but the fish was to die for. It was so crispy and crunchy and the tartar sauce was spot on. I ate it all.
I also spent a glorious afternoon at Spitalfields Market, which was an awesome craft fair and flea market where I bought quite a bit of artisan items. Think posters and photos and cotton dresses and cut paper cards. It was awesome.
I also had way too much fun at the airport duty free shop. The Heathrow airport is one of the nicest places I've ever been. It was spotless, it had showers and sleeping facilities and so much shopping I didn't even mind killing an hour there.
In all, it was a very busy trip that lacked a lot of sightseeing, but a work day in London is still better than a work day in suburban Chicago, because check out this view from my hotel: