I've no idea if it's changed, but when they were first released they only had Type Approval for speed monitoring in one lane. So if you entered in lane 1, and left the zone in lane 2 or 3 the camera would have recorded your speed but you could not legally be 'done' for it.

EDIT: Found this - "A Guide to Type-Approval Procedures for Automatic Distance/Time Speedmeters used for Road Traffic Law Enforcement in Great Britain by Dr SR Lewis states: "6.1.3 When image detectors are used, both shall be positioned to use the same angle of view to the same registration plate of vehicles passing the measurement datum points."

So unless the procedure for granting Type Approval has changed, swapping lanes will prevent you from getting a ticket.

However - Most Specs cameras are around roadworks, and they're there for a reason, i.e. there are blokes on the carriageway! So driving above the limit and swerving about probably isn't the wisest of ideas!