I always leave tarmac and tyres and a bit more when pulling up in a queue. Then you get some gloit pulling up as close as possible behind you
My old lady does that, and it gets right on my tits.
Back in 82 I was taught to leave a gap that allowed me to read the number plate of the car in front, but that doesn't always work with modern cars so I now adopt the tyres & tarmac approach.
I think its getting slower over the past 6 or 7 years.
Definitely.
People not using all lanes of a motorway how they're designed to be used (slow, overtaking, fast driving)
No such thing

30 years ago the rule was "Keep left unless overtaking", and I do believe that same rule applies today.
Any so called '4x4's' which aren't covered in mud (usually with a woman behind the wheel!)
We never felt the need to go mud plugging in ours, we bought it because it had 7 seats and I could get all my work tools in it along with any materials we needed for work (ie, 3m lengths of trunking). It also had the distinct advantage of traversing extreme weather conditions (eg, the january snow) with ease.
i use the lanes properly and it annoys me when people don't, especially people sitting in the middle lane! - MOVE OVER!
If someone is hogging the middle lane, you can always use your "fast driving" (L3 to the rest of us) lane to overtake before moving back to your "slow" (L1 to the rest of us) lane. 
That's how everyone should use it anyway as you said, but they don't and if people were'nt hogging the middle lane then i could use L2 to overtake and move back into L1, leaving people who want to drive faster to have the L3 lane, meaning faster moving traffic and safer for everyone. As far as pulling up close to the car in front i don't see the problem, i was taught the tyres and tarmac, but don't because if everyone did it, were there are lots of traffic lights you'd only get about half the cars through causing more traffic jams. If people payed attention you shouldn't get hit up the arse anyway and if the car in front breaks down, just get the 2 cars behind to reverse back a bit and hey presto! I'm sure they don't break down as much as the extra amount of cars makes worthwhile.
And i'm not talking stupidly close before some smart alec replies but 18 inches is fine as far as i'm concerned.