This site says that there are new digital based ones linked by ISDN on the M25 at various spots.

If what it says is correct, that there are two infra red ISDN cameras then they must have their speed limit set very high. I have driven through the camera section at over 90mph on most work days over the last 4 years (I used to leave home very early to avoid the traffic).
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Remind me to change my signature to something more interesting someday