If you have equivalent academic qualifications then you will be asked to provide a Letter of Comparability from UK NARIC before attending your first selection day. The Scottish educational qualification equivalents are detailed in the FAQ page.
The above (from the BA FPP website) doesn't appear to differentiate between foreign
equivalent qualifications and domestic ones. It only speaks of
equivalent. I'm not sure whether NARIC will
only provide a letter detailing foreign qualification equivalence, but it would seem silly if you had an equivalent UK-based qualification yet couldn't apply because you had no way of confirming to BA that it was indeed equivalent. And yet someone coming from somewhere else in Europe would have the ability to provide such a letter. Therefore, there must be a sensible solution to this
problem that isn't specifically outlined on the website. In all cases (where you don't have the A-Levels or other requirements specified on the website), I'd still recommend contacting NARIC anyway. This is an opportunity of a lifetime and it would be a shame to miss out on it over something as trivial as that. If NARIC can't supply a letter of equivalence then you will at least have a response from them to show to BA that you did contact them.
Fly-boi: the website doesn't specifically state that they don't accept BTECs. In fact, based on the above paragraph it implies that they will, as long as they are considered equivalent to the A-Level requirements. Looking at the (convoluted) UCAS points allocations, it looks to me like certain BTEC qualifications are just as good (if not better) than A-Levels. It would be incredibly asinine to be bureaucratic to the point of rejecting an applicant in light of this, and I don't think BA would. Moreover, if you consider that they are happy to accept a 2:2 degree; what they are clearly looking for isn't someone with A-Levels, it's someone with a demonstrated track record of being able to study at a reasonable level and pass. Any qualification as good as (or better) than the A-Level requirements should therefore (more or less) be considered equivalent.
Obviously the above is only my understanding of the situation and shouldn't be taken as fact. I'd always advise anyone to get the relevant confirmation well in advance of applying.