If I am travelling at the maximum permitted legal speed and I have my cruise on, thats the speed i've chosen to travel. If I need to move out to pass a slower moving car, I'll check its clear and if it is, I'll move out. I will then move back in when the opportunity presents itself.
I'm not going to speed up to 'get it done quicker'. I'm not going to slow down to 'tuck in behind' somebody who has chosen to travel slower than I have.
Why should I do either of things? Accelerating is a pointless waste of fuel. Slowing down... why?
I have absolutely no issues with people who wish to exceed the speed limit. But I won't break the law purely to accomodate them.
Similarly, if I make the decision to travel at a certain higher speed, thats my choice. It's not up to everyone else to get out of my way unless they are sitting in the outside lane for no reason.
Well obviously, but that goes without saying.
So your Motorway economy is just 4-5mpg better than mine - you in a diesel Civic, me with a 3 litre petrol saloon, I think that just made my point