I think that the way it is done in UK is almost okay, ie it sounds like in Portugal that these are central government provided regional test centres and while that might work well in densely populated areas of Portugal, in rural areas, people would probably need to travel many Kms to get their car tested.
Another frustration for me with the MOT test stations I use is that none of them offer timed slots - so as I said earlier, you are expected to turn up when they open and leave your car all day! Most MOT test stations in UK are just general or dealership garage workshops which have set up a small area for carrying out car testing, others are in local authority owned workshops that are set up to service local authority vehicles - and it seems like if you have then been certified to test vehicles, then you can not limit that certification to just your own vehicles, which is a bit silly but can mean that these MOT test stations have no interest to making money out of failing and repairing cars.
I'd guess that in UK at least, the workshop that has been certified to perform MOT testing only gets paid for ever car they test, so it is easy to see that most of them will most of the time make more money using the certified MOT tester servicing and repairing cars instead of testing car.