Since the R&F Scottish civ doesn't include any ancient Celtic features, there's still room for an earlier Celtic civ. Gaul is the best option, IMHO. For the leader, I'm an unabashed Vercingetorix fan. I expect the UU could be the Solduri or potentially their Auxilia cavalry (though the latter needs a better name). I've seen talk of the Oppidum as the UB, though I've seen other possibilities floating around as well.