Deal is good enough to host an Open and it probably will, some day, in a gesture to history not unlike the 2006 Hoylake decision. Before then, however, it will have to go through the normal dating process of hosting an Amateur, a Senior Open, etc. to see how it really plays (both on and off the course)under tournament conditions.
Getting back to the question, the best UK course not on the Rota is clearly Dornoch, ahead of a number of other fine courses including County Down, Portrush, Kingsbarns, Western Gailes, Rye and Deal.