One of the biggest causes of traffic jams on motorways is 'the rubber band' effect when one person brakes, the next brakes a little more and so on until the traffic stops completely … so often caused by fast moving vehicles passing a police / traffic officer / camera. Most people have settled on a speed of 80-90 so upping the limit to 90-100 would allow drivers to concentrate on the road rather than searching for police / cameras – all whilst still having the upper limit of 100 they should not cross.
Since many people who are comfortable driving at this speed already do so, it is unlikely the increase would have a significant impact on RTAs. The only significant impact would be on the income from speeding fines – but if we are to believe what we are told then this is not the reason for the speed limit and cameras anyway! In fact, it would free up some police resources since less would be required to enforce the limits so could spend their time more usefully on 'real crimes'.