OP, to answer your question in your first post, yes you were being a lane hog and yes he was being a twat.
"but how long is it ok to stay in the middle lane when there's nothing right next to you on the inside lane?"
If there's nothing next to you and it's safe to move in, then move in. It frees up a massive amount of the road and makes everyones journey safer and easier.
"Sometimes I stay in there longer than I perhaps should"
Then you are most likely being a lane hog and ought to move in if you can do so safely.
"but that's either because I can see that I'd only have to move out again in a minute because there's a slow moving lorry ahead"
No real excuse if it's safe for you to move in. You should move in when it's safe and move back out when you reach the lorry. If everyone did this then again, everyones journeys would more than likely be safer, quicker and easier.
You should be able to judge when this is safe and when it's not, and base your decision on that rather than on feeling there's no point because you can see another lorry further on down the road. If you can't judge when it's safe or not, or if you just decide you can't be bothered because you'll only have to move out again, you don't belong on the motorway.
"or because I can see a junction coming up and there will be cars filtering onto the the motorway."
It still might not be necessary for you to be in the middle lane. Cars filtering onto the motorway should be looking for a gap in the inside lane and driving to speed accordingly.
If there is a reasonable gap between vehicles in the inside lane nobody should need to move over to or sit in the middle lane well in advance of the junction. You shouldn't have to do their thinking for them at the expense of people already on the motorway. The safest thing you can do in this situation is leave a reasonable gap between you and the vehicle in front rather than block a lane you don't necessarily need to be in.
"I did that yesterday" - if you were safe to move in then you should have moved in. Everyone should have been driving to the conditions of the road so you might have been better slowing down to 50/55 even though you say you didn't want to, because from your description of the spray and poor visibility it sounds like you would have been safer at that speed.
Again, it sounds like you were both at fault. You admit you had stayed out longer than you should have and the van driver was probably doing the same. If you had both been moving in when it was safe to do so and moving out to overtake slower moving vehicles the situation you describe probably wouldn't have happened.
As it was, you were hogging a lane and he was driving like a twat, especially in the poor conditions you describe.
However this "I don't really understand when it's ok to be in the middle lane and when it's not" , this "He overtook me in the outside lane eventually, and I thought he should have just done that in the first place because that's what it's there for." and this "I really don't get it." make me think you are being unreasonable.
It sounds like you really would benefit from a couple of motorway lessons to make you a more confident and safer driver on the motorway. And the outside lane is not there so you can just sit in the middle lane for longer than you need to when it's safe for you to move back into the inside lane.
In poor weather conditions it's more important than ever to drive sensibly and use lanes properly. You admit you don't always do that and worse, you still think it's okay to force someone even further out into the outside lane just so you don't have to move in when it's safe or slow down when you don't want to.