2 things, the M3 does not have variable speed limits and there is no M35
The orange sign you refer to must have been an advisory matrix, and since you were coming from Dorset, I also assume you were close to Rownhams services. I am pretty sure that this sign always reads 50, regardless of traffic flow. Apart from one occasion when I passed it and it was reading 70!!!! That was helpful advice
I assume you mean the M25, which does have variable speed limits, and as far as I am aware there is no legislation that says that average speed cameras have to be on both sides of the road. The stretch of the M3/M25 which I think you mean always used to be 70 with advisory 50 limits, which was sneakily changed to a mandatory 50 at the same time the cameras went up. As far as I am aware these are live and enforcing.