With BA, it's either DBV or ZAG. I would suggest to book an open jaw ticket, fly to ZAG, get a rental car, spend 2 days in ZAG, drive to Zadar, spend a few days there, then to Split and DBV, fly back to LHR from DBV.

If you want to chill out for a while, I would suggest Lopud, a gem of an island just off Dubrovnik, where time seems to stand still. It used to be a leisure destination for the nobles of Dubrovnik during the Dubrovnik Republic age, and they built their summer palaces there. One of them today hosts La Villa, an adorable little b&b run by a charming couple and catering mostly to Brits. I'm not affiliated with them at all, I just had the fortune to spend a few days there in late September a few years back, and it was magnificent. Their website is at http://www.lavilla.com.hr/.