There's a pretty (in both senses) low bridge on NCR 51 where it runs along the river in Bedford. You certainly need to duck. I took this photo.
Ah yes - also in Bedford is this one http://www.cyclestreets.net/location/20730/with one normal side and a low-headroom one for when the river's in flood.
I passed under this a couple of weeks ago and wondered how many people get off thier bike to go under it. I stayed on the bike with my chest almost resting on the handlebars.
There's a nice video of Copermill Lane on YouTube: http://www.youtube.com/watch?v=LvzFD_OZTC4
One of the bridges over the Thames path on the south side -- I think it's Hammersmith Bridge -- has very low clearance.Here's someone's to-arty-to-be-useful photo: http://www.flickr.com/photos/jerrold/2561150140/
This link will take you stright to the bridge part of the video (3m5sec in)http://www.youtube.com/watch?v=LvzFD_OZTC4#t=3m05s
Agree with Nigel and Tim, but isn't there another 4'9" bridge in Bedford that's even lower than those two? http://www.cyclestreets.net/location/20729/
Wouldn't be a problem on my recumbent. Mwaahaha.