Yes we used Interhigh for DS for Year 10 & 11. But I'm away for the weekend so might not be around to answer questions. There's a couple of previous threads though, I'll have a dig and try to link in case they say anything useful.
It certainly worked for DS, he enjoyed it and got a good set of results. There's certainly an advantage in it not being disruptive - if one pupil is causing difficulties then the teacher can "silence" them and take them to another area to talk to them about what their problem is, while still overseeing the rest of the class, though obviously not actively teaching them.
I think it depends really if your dd learns well from a school "model" - there's set subjects, a set curriculum and timetable. You can't wander off at a tangent in the way that other types of HE allow. If she's happy enough with that kind of set-up then it might well suit her.
But you do still need to add a lot of extras on top of the academic stuff - socially it isn't the same as actually spending time with other children, though you have them around to interact with in class there is still a need for the rest of the social stuff. And there are no practical things like art, music, sport, so you would also want to add those on.
I'm not sure it's a choice I would have gone for in earlier years, not that I ever thought about it anyway as DS was fine in schools. But I'm not sure the advantages outweigh the things lacking for younger age groups. It was perfect for 2 years for getting IGCSEs though!
Just thought it would be worth listing all the British online schools here: Academy 21 Academus (not sure if they function as could not get any answer from them) Accipio (RIP) Briteschool First College Interhigh Periplus Education (they actually do offer all the subjects not only core ones as well as personal tuition which can be tailored to the needs of each particular child and is not that strictly timetabled)