I think it should be increased to 90 on 3+ lane motorways and A roads, still 70 on Dual Carraigeways and 2 Lane motorways (ie. M11)...
I also feel that Caravans and trailers over 250KG should be restricted to 50 mph...
Then you'll have a lane of caravans doing 50 MPH, a lane of lorries doing 60MPH, and a lane of dawdlers and slightly faster lorries doing 60.0076342 MPH, so nobody will ever get a chance to try out the new 90 MPH limit for cars.

If they do, they'll be cruising along at 90MPH when someone pulls out on them at 60.0076342 MPH without appreciating that some of us like to make reasonable progress and bang!
The main issue on our roads is that we have quite a lot of traffic, and they are used more efficiently if vehicles are travelling at the same speed. Adding different limits for different vehicles just clogs them up more. The status quo seems fine for towing vehicles and HGVs. Most loads are stable at 60 MPH and most modern vehicles can maintain that sort of speed towing a sensible load.
The problem with upping the main speed limit is that the "drive by numbers" culture that our "speed enforcement by post" strategy has promoted means that there is no longer the common sense in many drivers to choose a safe speed for the conditions, and an increase in the speed limit requires that.
I'm no expert but I would say the conditions do not often exist to allow cruising at 90MPH safely, certainly at busy times. I guess this will become the norm if the limit is raised to 80, though.
Kevin