OHLE▸ is raised to a slightly greater height than normal over level crossings, (albeit within the ambit of a normal pantograph) and that height is well above whatever the 'low bridge' definition is, 16' 6" apparently after some research. I think there might be some additional warning signs when wires are present, Google Street View should confirm either way. For example here at Waterbeach, north of Cambridge, there are signs showing 'safe height 16' 6":
http://goo.gl/maps/xUZQIt isn't a significant issue though, there are level crossings all over the
AC▸ electrified railway elsewhere, such as on the GE, the
ECML▸ and previously the
WCML▸ , although most of the latter have been bypassed as part of the route modernisation. But the latter was all about increased train speed and train frequency, nothing to do with the risk of high vehicles causing problems.
Paul