Hello 
my dd1 is 6, pretty severe ASD, and we have been using ABA with her for nearly 4 years now. she is at an ABA school fulltime.
so, your questions:
imo, I htink a few hours of ABA is better than none. it is more a mindset/technique/style/whatever you want to call it, so I find that I now parent in an ABA way. obviously dd1 gets mor eintenive. targetted ABA at school, but just the way we do things at home - it's ABA all the way.
DO NOT believe anyone who tells you it is all or nothing. we started out doing 12 hours a week with dd1 when she was 3 (minimally verbal, lots of stims - very withdrawn, etc). you do what you can do.
I don't have any experience of Autism PArtnership. I used Sean Rhodes as a consultant, and would thoroughly recommend him.
Price wise - it depends on who you go with, and what you do. eg, some providers set it all up, provide supervision, tutors, etc. this obviously comes at a price financially, but does alleviate a lot of the hassle of finding tutors etc. others, you woudl have a consultant who comes down maybe every couple of months ot oversee things, and then you find a tutor (gumtree, university student, local interested childcare worker) and you pay them the going rate - form a standard £8ish/hour untrained, right up to anything like £20/hour for a hghly experienced superviser-level (I owuld argue if you use someone at this level, you would ned less visits form a consultant, so it can be swings and roundabouts as to cost). COnsultants can cost in the region of £500/day - a full day working with you and your child, training up tutors, plannig out the next steps to take etc.
Re: finding a provider, ask around. see if you can find some families local to you, and observe their programmes. talk to a few providers, and see what they say in response to your questions. like any other area, there will be some you like, some you don't. try to sound out whether you can see yourself working closely with them - are they a good fit for what you want?
eg, I turned down working with 2 fullproviders as they wanted ot set up a fulltime programme only, and we wanted a part itme programme (both cost wise, but also just ot fit in with us as a family).
when you say you would want it on your ds' statement - as a home programme? in school? mixture of both?
LA are extremely reluctant ot fund ABA. you would have to set up, run, and prove worht of any programme befor ehtey will even consider funding it. you have to prove it is the only option for your ds. not the best option, but the only suitable option.
High-fnctioning wise, there is no reason why ABA will not work - it take swhatever targets your ds needs to work on, at whatever level, and works on them. but LA woudl seize on the high-functioning aspect as a reason t not fund, I suspect, and hide behind the fact that your ds is able to cope (in whatever way) in a MS setting (which I assume he is?)