From talking to (not very happy) friends in Scotland, it sounds as if Highers are at a lower standard than A-levels (an A is a C / D) but many schools don't offer many choices for Advanced Highers because not a lot of pupils take them?
I realise you can go to uni in Scotland with Highers only, but doesn't this put you at a disadvantage when applying for unis in England or elsewhere?
Also, quotas for Scottish students at Scottish unis ... I've heard there are some courses for which the only Scottish students they accept are those with significant contextual flags?
So basically, if you can't get into a Scottish uni, you need to take AHs to (realistically) apply to a good uni in England. But your school may well not even offer the AH subjects you are interested in. So you are stuck between a rock and a hard place, by the sound of it?