I work in a higher education institution and deal with student support and know a bit about the Student Support Regulations. I'm not such an expert on HND's.
The advice I'm going to give you below is based on a student living in England. The regulations are different depending on which country you live in.
The number of years funding you get is based on how long the course is (my experience is with degree courses), so if the course is 3 years in length you get 4 years funding. The '+1' year is to allow for retake years where the student does not pass. SFE do not fund repeat years (where you are retaking a year that you have already passed), so if the HND the student completes is equivalent to the first year of the degree, they will not fund the tuition fee for that year, but will do so for the remainder of the course. Even if you were to pay the fees for the HND yourself, I think this would still be the case. I can't remember whether or not they offer maintenance as there are cases where they do give a maintenance loan but not a tuition fee loan.
If the student then wants to go onto study a course at the same level (ie, another bachelor degree) they get no funding, and the institution is able to charge them the full cost of the course (the overseas fee), although this is at the discretion of the institution. This is referred to as the ELQ rule (Equivalent or Lower Qualification). The only exception to this is if the student receives a Disabled Students' Allowance (DSA) during their studies. They still don't get any student support, but are only charged the 'home fee' rate.
Private institutions can charge what ever they like. Students can borrow up to £6,000 from SFE towards tuition fees (but I presume only if the institution and course is registered with SFE). I don't have experience of this so don't know about maintenance support.
If you can call SFE during the day it normally takes about 5 mins to get through to a 'real' person if you call in the morning. The other alternative is to contact the institutions you are looking at and explain the situation. They should have a good understanding of the different pathways that students come from and what finance will be available.
So many times students I come across seem to assume that funding will magically be available to them so I am glad you are thinking ahead about this. If you get an explanation that you don't understand, keep asking until you do because the regulations are not easy to access and understand, and it's important to make sure you are clear of the implications will be funding wise before making any firm decisions.
Hope what I have written here is of some help.