In believe Knockhill was built from scratch while Ingliston was on 'something existing' (in that case the roads of a showground) and the other Scottish venues were all airfields.
I'm not sure whether to agree about Brands. It depends on the time period you work to. Originally it was a field where motorcyclists ran grass track races. Arguably that changed it from a field to a sporting venue. Then the grass track was paved and it became a race circuit. So over a 30-40 year period you could say that it was built 'from scratch'. If you take 'from scratch' to mean taking a greenfield or brownfield site and creating a circuit in one fell swoop like Abu Dhabi, Bahrein etc. then I won't argue. I don't know the detailed history of Mallory or Lydden Hill; but I suspect the same "over 30-40 years" argument might apply.
As you say the Brands situation is bit oblique, it was a venue for other types of motorsport long before it became a car-racing road course, so the paved circuit wasn't cut from virgin earth.Certainly as a car venue it was an adaptation of something already in place.
Mallory was a horse trotting track, then a motorcycle grasstrack before being paved so the same applies.
Lydden was an early stock car oval that grew and was later paved. That's closer to being scratch built but it wasn't purpose-built as a road course for RAC sanctioned motorsport - although I am being picky there, I agree.
Knockhill sounds like it may well be in a unique situation within British motor sport - the only scratch built road course we have ever had - in over 100 years, 50 of these as probabaly the 'centre of the world' for motor racing.