So you've got ten nights between leaving Prague and flying back to London.
You want to visit Cesky Krumlov, Vienna, Salzburg, Budapest, and Slovenia. (Is there some reason you are going to Slovenia? I love it and highly recommend it - just wonder why that is your end point.)
June 28 Cesky Krumlov
29 Cesky Krumlov
30 Vienna
July 1 Vienna
2 Vienna
3 Salzburg
4 Salzburg
5 Ljubljana
6 Ljubljana
July 7 Ljubljana
I don't see how you could fit in Budapest. There are shuttle services to get from Cesky Krumlov to Vienna or Salzburg. Take the train from Vienna to Salzburg. Doing both Vienna and Salzburg means a bit of a detour and a few hours extra travel time (plot all your cities on a Google Map, with driving directions, to see where they all are). You can take a train from Salzburg to Ljubljana - a little over four hours.
Wizz Air has a direct flight from Ljubljana to London on July 8 but it's already pretty pricey ($125 pp - plus fees). Easyjet has two flights that are already $200+. Busy weekend? Or just high season prices? Not sure - I don't travel at high season...
You would save money flying out of Venice instead the same day - plenty of fares late day / early evening for under $100 pp. Ryan Air has a flight to Southend airport - not familiar with it but only $33 pp! (plus fees). You could take a shuttle (e.g. GoOpti) from Slovenia - probably a three hour ride to the Venice Marco Polo airport.