I might wear my HRM on my commute tomorrow morning just to see if I can put some numbers on it. I probably do exceed 120bpm but I'd guess I very rarely go above 150bpm unless I'm late and in danger of missing my train.
My lunchtime runs usually average ~170bpm for 35-45mins.
I have tried running to the station in the morning but it's not really practical in winter - dressed in just running gear, I get very cold on the train (I can get away with wearing/carrying heavier clothing when I'm on the bike). I'll definitely do it more in the summer though.
You do see a lot of people running to/from work in central London. Not as many as cycling, obviously, but on the other hand, you don't see many people going for a lunchtime bike ride.
If I'm doing my long ride after work, I do often push quite hard but I think because I'm conditioned to cycling, I have to push harder on the bike to get a benefit than when I run. Hence running is a better workout for less effort, for me. YMMV.