Where are there minimum speed limits?
Mostly tunnels, though a vehicle on a motorway must legally be capable of 40mph this is a bit different than being required to do so.
Is failure to comply with them really a problem?
Yes, it's an offence as well as being inconsiderate of other road users. I don't know under precisely which law, it may just be Driving Without Due Care and Attention or similar. Obviously if you're required for safety reasons to reduce speed (e.g. because the vehicle in front is going more slowly) that would be a defence.
If you're not happy, in normal conditions, to drive at the minimum speed limit posted for a given road, you should choose not to use that road.
As for raising maximum speed limits as mentioned above: do truck drivers now leave decent gaps so one may comfortably use the first lane to join and leave the motorway?
My experience is that if I accelerate to 56mph (lorry limiter speed) lorries will always let me in, particularly in cases of known short slip-lanes such as it used to be leaving Leicester Forest services southbound. They are generally quite considerate. This is one reason going to 60 in roadworks is an improvement - it could be hard to join if you wouldn't go over the limit to do so. Yes, lorries shouldn't have been speeding, but road safety measures need to be based on what drivers do, not what you want them to do, and cameras aren't generally accurate enough to enforce at 50.00001mph even if you wanted to.
If you aren't comfortable to accelerate to 56mph by the bottom of the slip-lane (or around 70mph if there aren't any lorries and conditions are normal) then the motorway isn't for you as you will be posing an unnecessary risk to other drivers by failing to make progress. If you wish to drive at 50mph or less when the limit is higher, the parallel "regular" road is where you should choose to drive.
If you are trying to join motorways at below the prevailing speed (and I see surprisingly many people doing it*) you are making things hard for yourself and more dangerous. Try it some time, you will find the experience much easier. It's one of those cases where being unduly nervous is self-justifying, because the experience when you're unduly hesitant is so bad, whereas if you aren't then it's easy. If you're genuinely not confident to join a motorway correctly, some motorway driving lessons with a good instructor might pay dividends.
* If I do think someone is going to do that, I tend to hang back on the slip-lane so they get well ahead before accelerating to the prevailing speed myself, so at least I'm not at risk by their actions.