I've got no personal experience of riseholme sorry but lots of friends kids and friends of my kids did go to equi/agricultural college so I can contribute from those points of view.
I do have slightly mixed feelings about it - from the point of view of career prospects directly it's not necessarily the best, in theory/on paper anyway, for academically able kids they're generally better off taking the traditional a levels and university route, for those that want to pursue a professional career as a rider (very competitive/difficult obviously) they should really look towards working pupilships and opportunities to be competing or at least working on a high level yard and continuing to compete in their discipline independently, and for those that want to work as a groom or instructor or similar apprenticeships are a more direct route to employment, particularly if they can do their BHS exams as part of that. Or the British racing school courses have very good employment options afterwards. I have to say that the majority of the kids we know who went to equine colleges (either at 16 or 18) don't now work in the industry, or do but didn't get the jobs they have because of college if you see what I mean. So you could say what's the point of the courses then.
But that being said that is taking a very narrow and limited view of what education for 16-18 year olds is 'for', it's not just directly about getting a job or qualifications that lead to a job, for me there's a really big piece about self development, building confidence and skills, finding yourself for want of a better word and also just having fun and making friends which I know can never really be captured in policy or strategy - they've got an awfully long working life ahead of them as teens so it's a pity to pigeon hole them so early into only things that are directly career building. The environment at agri/technical colleges can be so much more suited and positive for the kids that it works for than school or general college - more practical, outdoorsy, based around their interests, everyone there has self-selected as opposed to it being forced on them etc. Even if at the end of the course the kids decide quite understandably a career in horses isn't for them and move on to a different job or other further education, if they've come out with better focus, some qualifications they probably wouldn't have got otherwise and generally better confidence and life skills then I'd chalk that as a win. And TBH I think a determining factor is by the time they're 16 if it's what the they really want to do you have to weigh that heavily in it's favour as they'll be more motivated compared to maybe what you as a parent thinks would be better but doesn't interest them at all...
That being said I also have to say from my anecdotal evidence while some kids do have a good time it's not 100%, you do have to ask the right questions about whether it is the right place for them, socially for instance it may be better/easier than school but it's not guaranteed and that could become a big issue if they'll be boarding/residential. SN support as well, some colleges are excellent on this and some are frankly poor. Safeguarding etc - I'm sure you've looked into all this but I guess you have to avoid getting swept up in DD's enthusiasm and make sure you've done the due diligence?