What's happened in the UK is that a standard engineering degree (BSc, BEng) was always three years of study (as it was when I got my BSc), although they had to cram a lot of material into that three years to make it all fit. In my first year we had timetabled lectures, tutorials and labs from 9 to 5 every day of the week, so that all assignments, project write-ups and tutorial study had to be done in evenings and weekends. It was a punishing schedule.
Around the time I was studying, some universities started to offer an MEng degree with four years of study open to the most able students. Originally, this was seen as a "super degree" (not a Masters, just an advanced undergraduate degree).
Then gradually sentiment changed in the engineering world and the MEng started being seen as the regular degree, and the BEng as an inferior degree for weaker students. The professional institutions changed their requirements. Where originally a three year BEng would satisfy the academic requirements for professional registration it was no longer so. Now a four year MEng became the minimum.
In theory, therefore, UK students get six years of study for their MEng degree: two years of foundation study in the sixth form and four years of undergraduate study at university--although there are many complaints (perhaps justified) that the standard of sixth form A-level study has become weaker today. There is less problem solving, fewer open-ended questions and more directed solutions, multiple-choice and continuous assessment.