Not sure if I can make it clear but I'll have a go -
RHS level 2 qualifications are theoretically at the same level of study ( going into topics in the same depth) as NVQs at level 2 - which is generally thought of as the O-level/GSCE equivalent, Level 3 as A-level.
The reason all the RHS courses changed their structure a couple of years ago was so that they fitted into the same framework ( the QCF) as all these other qualifications. The old National Certificate and Diploma in horticultural colleges made the same changes, and are now'Level 2 cert/dip', 'Level 3 cert/dip', where the difference in the qualification title relates to the length not the depth of study; and NVQs are work-based qualifications at level 2, level 3 or even level 4 following the same structure.
The difference between an RHS course, which is part time, and a college full time course will be the number of hours you put in, not the depth of your knowledge. However the RHS does pack an awful lot into the hours for you to study.
A level 2 NVQ is essentially a much more hands-on qualification, proof that someone can dig and prune and plant, and the closest equivalent with the RHS would be the RHS level 2 practical - though I think that demands wider plant knowledge. ( I'm not a work-based learning specialist.) But many employers like RHS theory qualifications, because they show a willingness to study off, as well as on, the job.They are also harder to achieve than assessment- or coursework-based qualifications!
Your earlier question about HNC/HND qualifications - these are level 4 or 5 courses, full and part-time equivalent to foundation degrees and with fees to match because they are "HE". The expectation from next year is that you have to get a student loan to fund even part-time HE, it will be so expensive. New high fees are going to hit a lot of colleges where HNC/HND garden design was a good option for mature career changers thinking of changing to horticulture.
There's nothing to stop anyone from a non-horticultural background starting in at HNC level if they have the right qualifications for a college to accept them, but these courses don't fill in all the level 2 and 3 skills and plant knowledge you still need - at one college where I worked we used to recommend Foundation degree designers did level 2 RHS as well, so thatb they knew what they were talking about.
Does that help at all?