The following confirms that you can change lanes and will still be caught.
SPECS cameras operate as sets of two or more cameras installed along a fixed route that can be from 200 metres (660 feet) to 10 kilometres (6.2 mi) in length.[1] They work by using an automatic number plate recognition (ANPR) system to record a vehicle's front number plate at each fixed camera site. As the distance is known between these sites, the average speed can be calculated by dividing this by the time taken to travel between two points.[2] The cameras use infrared photography, allowing them to operate both day and night.
There is a popular misconception that the Home Office has approved the SPECS system for single-lane use only. According to this theory, a motorist can therefore switch lanes between cameras and claim non-approval to avoid prosecution for speeding.[3] However the marketing director of the manufacturer, Speedcheck Services Ltd, has stated that this theory is "categorically untrue":[4]
“ Until recently, the only HOTA available applied to cars maintaining their lanes. However a new test schedule was carried out last year, which means that average speed checking can be applied even where cars change lanes. ”