To think it’s better to drive at 31 than 25?

I wouldn't know, I wasn't there and I haven't seen the road. But if everyone is going so much slower than you, it might well be you that is wrong.

My personally favourite is when people drive 40 in a 30 and then continue to drive 40 in a 60. Makes no sense lol

YABU. You should drive according to the conditions of the road, not what they speed limit sign says. If everyone else is driving slower than you, that might be for good reason. A few mph slower won't make much difference to your journey time, and if it does, set off a few minutes earlier.

You are right.
The slow driver in the outer lane was at fault.
OP simply had to continue driving in the inner lane (being alert to the other driver) and no problems or delay were incurred.

You might find that it enable you to relax a bit and enjoy the journey.

My mum used to have a great saying about driving and rushing about,
"Better five minutes late in this world than 20 years early in the next."

You were driving on a clear road.
You are correct that the driver doing 20 in the outer lane was incorrect but as the inner lane was clear your journey wasn't affected.
Your hyperbole is amusing.
I'm guessing you are bored and keen to get a rise.

From what I learned on my speed awareness course (cough - it was ten years ago) 34 mph makes quite a difference to injury/likelihood of survival if you hit someone, v 30mph.

That said, going along at 31/32 is likely to be still under the limit as you are likely actually doing 29mph.

What is more annoying and dangerous than drivers going really slow is drivers going slow because they are faffing about and not paying attention to the road. I can see them turning their heads and chatting to a passenger and not looking, fiddling with a phone or something on the dashboard.

Or they don't pay attention to road signage. I'm wondering how many drivers do not have their eyes tested or change their glasses as often as they should as I think they simply don't see it.

I was behind one guy doing 22mph in the 20mph limit the other day and he clearly did not notice when it had changed to 30mph or the subsequent signs and carried on pootling along.

Often you get the other way round which is worse. They are doing 40mph in a 60mph limit (on a road where it is possible to do close to 60 safely in good conditions) then carry on at 40mph.

My instructor told me I'd fail my driving test for not consistently driving close to 30 in a 30 zone, assuming the conditions are appropriate. Doing 20 in a 30 would be considered unsafe driving, unless there was a good reason for it.

This isn't quite the right interpretation. Your actions should not cause another driver to swerve or brake.

Actions in this context would mean things such as pulling out of a junction, changing lane and so forth. Manoeuvres. Not going slower than the speed limit in a lane in which you are already established.

If your interpretation of the theory was correct, then tractors, lorries, buses and any other vehicle which isn't fast or which may be speed limited would be illegal on the roads.

31 is around 28mph in reality. Speedometer are designed to over read.

On a motorway, your 70mph is closer to 65.

If so many people are doing something you think is wrong, maybe it's you?

I drive to speed limit or to safety, whchever is lower. The number of people getting agitated around me because they are busy and important is growing every year. Sometimes I have "local knowledge" that there is a temporary (2 years so far!) reduced speed stretch around the next bend and it's always a pleasure to let the racers overtake at mad speed and get hit by the flash of the camera. Sometimes I use natural breaking before I turn off a road so as not to waste fuel. Pootlers can be frustrating to be stuck behind, but better we are all alive, no? Best to drive near the correct speed, but the snails seem safer to be around than the angry racers.

I only took my test this year at the grand old age of 52. Passed first time, was very shocked lol.

my instructor was very insistent that I don’t cause other cars to slow down at the speed limit as it makes me the danger.

also taught me how to undertake, there are questions about that in the theory as well.

After doing a speed awareness course last year, I absolutely go the speed limit or less now.

Not only for safety but the entire class was filled with people (including myself) who were constantly in a rush.
One man was almost spitting because he was so angry that people drive slow and I thought I don’t want to be like him.
I am not miserable like that, I love my life and I’m very happy.

Like why are we stressing ourselves out so much.
The world is so fast paced and it’s not healthy to be rushing around to get to places all of the time.

If I am late, then it’s my fault and I should have left earlier.
Me speeding or getting worked up over it isn’t going to help.

Now I turn my music on or listen to an audiobook and enjoy the luxury of having nothing else to do but drive.
Once I get to my destination then I can go back to rushing and trying to juggle 20 things at once.

If the lane is going to be turning right then maybe they are just slowing down rather than having driven at 20-25 the entire time? Or see no need to zoom about the place and slam on brakes. I've driven in Sheffield loads so am familiar with the type of layouts you're talking about, I agree with the PP that if you're slowing down when someone is in a differently marked lane then actually you are the one at fault.

Technically undertaking is any passing on the left.

Undertaking in itself is not an offence, as there are many occasions where it's necessary, e.g. congestion, filter lanes.

Undertaking can constitute dangerous or careless driving. If you are observed passing on the left and the police feel you have done so recklessly, that is when you may get pulled in for it.

You have one extreme of someone weaving and zipping in and out, and yes that is very likely to be charged if caught. But any undertaking not on congestion / filter lanes etc could result in a fine or points if it is felt your standard of driving fell below that required. There have been successful prosecutions of undertakers who have done nothing but stayed in their lane and passed traffic in lane 2 on the left.

From your subsequent descriptions it sounds like you have a fundamental misunderstanding of the layout and how to drive it anyway.

Even though the two lanes are adjacent, they are independently marked for separate directions and what someone is doing in the other lane is not relevant to you, so you could have passed at any speed you like, not felt beholden to stay at your hated 20mph.

I really object to the suggestion that drivers going slowly and not posing a risk to others with their driving are driving dangerously because other drivers can't exercise a bit of patience.

Proceeding in your own lane isn't undertaking, by the way, it's fine to go at 30 in the left land and pass someone doing 20 in the right. It's looping about and zigzagging in and out of lanes on people's blind side that's dangerous.

you can if the alternative is to do something dangerous

Then you either have very poor eyesight or very poor hazard perception, that you must brake so harshly so quickly to go from 30 to 20 to slow down on a clear road with a visible obstacle.

If you were driving within the speed limit in the inner lane you are permitted to continue past a car doing less than the speed limit in the outer lane.

No "slamming" necessary.