There are several air carriers which fly to Croatia or to Trieste in Italy, very close to Rovinj.

Croatia Airlines offers return fares from £148 and flies from Gatwick and Manchester to Pula, a town only half an hour from Rovinj, and  from Heathrow to Rijeka, two hours from Rovinj.
Easyjet flies from Luton and Bristol to Rijeka.
FlyGlobespan flies from Glasgow and Edinburgh to Pula with fares from £79 return and other budget airlines include Holiday Options who offers fares from £99 return including all taxes, flying from Gatwick and Manchester to  Pula.
Thomsonfly flies from Doncaster, Sheffield, Gatwick, Manchester, Birmingham, Bristol and Bournemouth to Pula.
Starting 31st October, RyanAir flies from London, Stansted to Pula. Fares start at € 2.99.

Of course, you can fly to Trieste or Venice in Italy with RyanAir and rent a car to reach Rovinj.

Since Pula is very close to Rovinj, actually, Pula is the capitol of Istria county, it is the most convenient to fly to Pula, and then come to Rovinj by bus, rented car or in our arrangement.  

Regarding accommodation, we can arrange it for you. Depending what you prefer, you can stay in 3-4*  hotel for the price between 30 - 90 € (subject to touristic season), or private accommodation with average prices between 20 - 40 €.
Since Rovinj is famous touristic destination, the quality of accommodation is very high and you can really enjoy.
If you would like to know more about Rovinj, here are few useful links:

http://www.istra.com/rovinj/
http://www.tzgrovinj.hr/eng/home.asp
http://www.rovinj.net/
http://www.rovinj.info/
http://www.world66.com/europe/croatia/rovinj

If you would like to hear our patients' experiences from Rovinj, please let us know.