Oh, I don't think motorways are that bad. IMO, even new drivers should just get on with it. Use caution, but don't hesitate too much.
Just pointing out that practising when there's no traffic won't prepare you for a busy motorway, and that sometimes it's not possible to speed up to 70 and merge seamlessly, as some would have you believe.
I'd rather practice during the day, but not rush hour, when there's some traffic but not so heavy that it's difficult to merge.
For new drivers, if you can't see where to merge easily, it's best go at a slow speed at the start of the slip road and build up speed when you spot an opening (or a generous driver on the motorway) and then merge. Ignore the annoyed drivers behind you.
For bends, brake/reduce speed before it starts, reduce the gear, engage lower gear. You can control the car more easily during the bend, then speed up from the bend. I think it's a terrible idea to practice at speed during the night with the idea that you can go off your lane. You're supposed to be practicing for keeping within your lane, not a rally or F1.
imo in your situation its more likely to be the case their speedometers are showing a lower speed than yours, and/or they are the type that seem unable to maintain a fixed speed rather than such a large number of people wishing to drive too fast.
Oh, when I'm going at 5-10mph above the limit (good roads, very light traffic, good visibility), and they go past me, it's not because they're crap at maintaining speed. 
I understand that slightly slower drivers have different speedometers or are afraid of going over the limit. I'm fine with that.
As others, I may be concerned when they're driving more than 10mph below the limit, but I don't know why, so if I get impatient I know it's my problem. I'll just patiently wait until I can overtake, or pray that they take a different road.
I don't see why they should constantly stop to let others through, unless they're at a true snails pace and are too big to overtake. If we are all maintaining a decent average speed, it's only a few minutes difference. And if it's a long journey, chances are you'll be able to overtake or take different directions soon enough.
As someone mentioned earlier, you may overtake and never see the car again, but in all likelihood you'll only be a couple of minutes ahead of them. So, there's no point in taking risks or getting angry.
PS - on motorways, I tend speed up past a group of cars. It's often caused by a slower vehicle, most cars tend to drive too close to each other and often switch lanes without sufficient care. I'd rather go past as fast as I can and reduce back to a lower speed further ahead.