I agree. Speed limits on the motorway being one of those things where discretion is best.
And I disagree with directhex's attitude that going over 70mph, particularly in lane 3, is bad. It does just hold the traffic up if you overtake someone doing 66mph in the middle lane, as it takes far longer to do it and causes those behind who wish to make more progress (And no they're not all pushy BMW drivers doing 120mph, mostly mere mortals like dangel and I for instance who are doing 80 or 85mph safely with plenty of awareness and space around us) to have to brake, etc. which has a bad knock-on effect.