It's nothing like BTEC, at all, whatsoever, etc.
I've done two BTECs in my younger years - performing arts & applied science - and I've just started Access to HE Social Sciences.
The course - level 3 - is done over one year instead of two. The assessment process is different - at least in my experience - in that you don't keep getting work handed to back to you to improve on it; if you get a merit when your assignment is assessed, then that is your grade. You only get your work handed back to you if you've missed something very important - like if one of the assessment criterion is completely missing. In BTEC,you could get the work back several times in order to achieve distinction after distinction, etc.
Access to HE is more suited to adult learners (mature students). In an intake of 20 social sciences students, my college has just the one teenager (18), who has been given special dispensation due to having a six month old. My college generally only takes people 19+ on the Access. In fact, the average age is about 35 across the entire Access area (six pathways) - which, apparently, is older than previous years. I'm an anomaly. If I were older, I think the average age would be older, also.
Access is also very academic. Heaps of written assignments, exams (we actually do time-constrained essays at my college, but I've heard that others do actual exams). BTECs are very practical. Universities accept them as a qualification, but I never really understood why. I never felt like I was being prepared for academia until I started Access to HE. Especially with all the attempts given to get a distinction.
This year, I have 3 modules (psychology, criminology & sociology) and 60 credits (45 taught, fifteen credits per module + 15 untaught, as our college is only allowed to teach 45 credits), all at level three, even the untaught units - the untaught units, for me anyway, are core skills. Some colleges, apparently, do core skills at level 2. Mine doesn't, hey ho.
BTEC courses might expect essays for all assignments (or lab reports in science-based subjects), an Access course may use different assessment methods - this year, in each module, I have at least one essay, one booklet, one solo presentation, one research project and one time constrained essay (exam conditions, but not an actual exam - for example, in Criminology on the 4th of November I will have 100 minutes - i.e. one whole class - to answer five questions essay-style, and that will constitute one assignment). One of my psychology units has an essay AND a booklet. Either sociology or criminology requires one assignment be both a powerpoint presentation AND a research project.
I reiterate points - Access to HE is hell of a lot more intensive (some colleges offer it part-time over two years, yada yada, but not many from my research). Hell of a lot more academic. Hell of a lot more strict with grading. It is definitely for students who are serious about going to university. Plus, they only accept "mature" students, with the occasional exception (the 18 year old in my class - but she is a parent). Lots of colleges also only provide evening classes. Never understood why. Especially when most of their students have families, lots I have come into contact with are also single parents with little support.
I'd go as far to say that the only similarities are the grades you can receive - i.e. Pass, Merit, Distinction - even if the way you are actually graded is heck of a lot different. And that you do lab reports if you study science subjects.