I've done a number of overseas shops and there are some great restaurants and bars available in London from US companies.
While it's probably a grey area, working for a US based company and reporting on an overseas experience is not specifically "working" abroad. You could technically be coming back and doing the reporting from the US. Domestic workers are regularly sent overseas for business trips without work visas.
That said, I would be hesitant to bring it up at immigration. I'm usually playing the part of a tourist when I am there, so I keep that role up whenever possible.
Years ago, I made a quick trip to Canada for a hotel assignment and got the MSC to pay for a repositioning cruise that dropped me in Vancouver as my transport, since it was cheaper than flying. Apparently cruising by yourself with a lot of computer gear and a ticket home 2 days later is suspect, since I got pulled aside for further interrogation. At that point, I decided it was best not to lie and came clean that I was mystery shopping a hotel. They seemed fine with that answer after I showed them my assignment paperwork and let me go.
Ironically, I ran into the same immigration officer when was leaving the country. He came up to me while I was waiting in line and asked, "How'd we do?" so they didn't seem to care.
I also have a story about a friend who showed up in Paris with a guitar as his carry on and told immigration he was coming in to play a gig with a band. They sent him back home on the same plane and denied him entry..