Not sure if its still the law to do so, but in the not-so-recent past the siting of a speed camera (of any type) HAD to be preceeded (withim a certain distance on the stretch of road in question) by a sign denoting 'speed camera ahead' (you've seen them before). Similarly for Bus Lane Cameras and ones covering box junctions (none here).

I agree with the other response in that its more likely to be a general CCTV camera (odd looking though, even for a new one) or polution/weather monitor. My bet is on the former, especially as its mounted rather high up to get a clear view of number plates at a reasonable distance, and whose view could be easily obscured by other vehicles.

[update] - just found a newer photo from Google Street view, which seems to show that its a CCTV camera - the actual 'domed' part just wasn't installed at the time of the photo on the other site. See:

www.google.co.uk/maps/@53.6036012,-2.1735786,3a,72...n