I always have trouble using the Doak Scale, especially since I don't own the book and don't have the entire scale memorized. However, from your list Sean there is a dramatic variation in the golf courses that Doak listed uniformly as "6." For example, he rates both Deal and Worplesdon a 6, but I think these two courses are not even remotely in the same league. While Worplesdon is a fine layout, Deal is much stronger all the way around.
Therefore, I find it difficult to compare different courses on the Doak Scale, especially since Doak's own rankings are inconsistent in my mind. Seaview Bay is not as good as Deal or Worplesdon. Yet that does not mean it isn't still worth playing. Seaview Bay is a classic, public-access layout that contains some great design features (cross bunkers, gathering bunkers, fall-away greens, great short fours, an intimate routing). If you are a classic course buff who is interested in the work of Donald Ross or Hugh Wilson, Seaview Bay is an excellent course to see. If a course that is under 6,500 yards is inadequate for you, than go to one of the many modern layouts in the area.
Is Seaview Bay an executive course? As Kyle Harris pointed out to me, any course under 6,000 yards is defined as an "executive layout." Seaview tops out at 6,300 yards, so no, it is not an executive layout. There are several great courses in the UK (and a few in the US) that might be considered "executive" layouts in the technical sense.